Improve decoding performance.

[sciencesakura]

Convert character codes to string in a single operation, rather than building the string incrementally.

before modification:

 ✓ src/index.bench.mts > MUtf8Decoder.decode > ASCII text 1866ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  138,611.30  0.0064  0.3600  0.0072  0.0072  0.0102  0.0129  0.0169  ±0.26%    69306
   · decode 5K length string    13,134.21  0.0683  0.3238  0.0761  0.0761  0.1715  0.1915  0.2576  ±0.48%     6568
   · decode 50K length string      781.09  1.1058  2.2982  1.2803  1.2347  2.1919  2.2186  2.2982  ±2.14%      391

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Latin-1 text 1851ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  133,635.72  0.0067  2.6423  0.0075  0.0075  0.0098  0.0119  0.0185  ±1.05%    66818
   · decode 5K length string    12,589.96  0.0713  0.3823  0.0794  0.0790  0.1731  0.1864  0.2431  ±0.46%     6296
   · decode 50K length string      754.70  1.1506  2.4067  1.3250  1.2599  2.3257  2.3311  2.4067  ±2.30%      378

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Asian text 1840ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  110,335.96  0.0081  0.3298  0.0091  0.0089  0.0156  0.0177  0.1103  ±0.49%    55168
   · decode 5K length string    10,826.64  0.0839  0.4914  0.0924  0.0893  0.2458  0.2576  0.3631  ±0.73%     5414
   · decode 50K length string      673.34  1.3122  2.4855  1.4851  1.4170  2.3558  2.3750  2.4855  ±2.09%      337

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Emoji text 1844ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  111,678.43  0.0080  0.3261  0.0090  0.0088  0.0124  0.0168  0.1117  ±0.48%    55840
   · decode 5K length string    10,886.18  0.0836  0.3125  0.0919  0.0892  0.2414  0.2483  0.3017  ±0.67%     5444
   · decode 50K length string      666.79  1.3056  3.0636  1.4997  1.4367  2.4745  2.5018  3.0636  ±2.22%      334

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Mixed text 1839ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  119,653.83  0.0073  0.2694  0.0084  0.0082  0.0153  0.0168  0.1213  ±0.51%    59827
   · decode 5K length string    11,678.15  0.0780  0.4179  0.0856  0.0838  0.1895  0.2087  0.3173  ±0.57%     5840
   · decode 50K length string      658.66  1.2233  3.6836  1.5182  1.3851  3.5177  3.5536  3.6836  ±3.97%      330

after modification:

✓ src/index.bench.mts > MUtf8Decoder.decode > ASCII text 1984ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  631,249.25  0.0013  0.6348  0.0016  0.0016  0.0024  0.0031  0.0106  ±0.52%   315625
   · decode 5K length string    66,023.80  0.0133  1.6001  0.0151  0.0148  0.0223  0.0291  0.1111  ±0.77%    33012
   · decode 50K length string    3,699.06  0.2357  1.4752  0.2703  0.2538  1.0566  1.1133  1.2945  ±2.12%     1850

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Latin-1 text 1939ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  460,527.54  0.0017  0.2494  0.0022  0.0021  0.0035  0.0041  0.0159  ±0.51%   230264
   · decode 5K length string    50,625.04  0.0170  1.8923  0.0198  0.0191  0.0353  0.0982  0.1172  ±0.88%    25313
   · decode 50K length string    2,298.99  0.3648  1.6079  0.4350  0.4136  1.3263  1.4626  1.5747  ±2.45%     1150

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Asian text 1887ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  274,637.11  0.0030  0.1704  0.0036  0.0036  0.0050  0.0065  0.0894  ±0.46%   137319
   · decode 5K length string    28,562.43  0.0306  1.4384  0.0350  0.0337  0.1142  0.1183  0.1561  ±0.78%    14282
   · decode 50K length string    1,293.43  0.6591  1.7620  0.7731  0.7301  1.5647  1.6226  1.7620  ±2.20%      647

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Emoji text 1885ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  274,088.16  0.0031  0.3078  0.0036  0.0036  0.0054  0.0067  0.0900  ±0.48%   137045
   · decode 5K length string    28,172.99  0.0309  1.4373  0.0355  0.0338  0.1173  0.1223  0.1745  ±0.81%    14087
   · decode 50K length string    1,284.75  0.6536  1.7045  0.7784  0.7346  1.5826  1.6540  1.7045  ±2.32%      643

 ✓ src/index.bench.mts > MUtf8Decoder.decode > Mixed text 1908ms
     name                              hz     min     max    mean     p75     p99    p995    p999     rme  samples
   · decode 500 length string  356,024.68  0.0022  0.2942  0.0028  0.0028  0.0049  0.0055  0.0168  ±0.50%   178013
   · decode 5K length string    37,972.70  0.0233  1.3814  0.0263  0.0259  0.0420  0.1098  0.1268  ±0.71%    18987
   · decode 50K length string    1,664.44  0.4829  3.8879  0.6008  0.5634  1.5879  1.6358  3.8879  ±2.87%      833