Extract UTF-8 valid length position to Bitstring class

[mward-sudo]

Extract duplicated UTF-8 validation logic into a centralized method in the Bitstring class, eliminating code duplication across multiple Unicode normalization functions.

• Validate UTF-8 byte sequences
• Finds the longest valid UTF-8 prefix
• Returns pos
• Handles edge cases: overlong encodings, surrogates, invalid continuation bytes

To-do

• Extract UTF-8 sequence length detection to Bitstring class #705
• Extract UTF-8 code point decoding to Bitstring class #709
• Extract UTF-8 continuation byte validation to Bitstring class #711
• Extract UTF-8 code point validation to Bitstring class #712
• Extract UTF-8 sequence validation to Bitstring class #713
• Extract UTF-8 truncated sequence detection to Bitstring class #714