Support for `undefined` type

[sergey-shandar]

It may require some value restructure to make undefined/void 0 equal to 0u64. For example,

tag = 0b0000_0000_0000_0 is an extension:

• v = 0b000_0..0 is undefined
• v = 0b010_0..X is bool
• v = 0b100_X..X is object
• v = 0b101_X..X is array
• v = 0b110_X..X is string
• v = 0b111_X..X is bigint

If tag != 0b0000_0000_0000_0 then return bitwise not of value as f64: Number(f64.from_bits(!value))