The number of bytes this string has for storing UTF-8 characters.
Does not include the null-terminator byte.
A setter is included for conformity with INativeList<T0>, but FixedString128Bytes.Capacity is fixed at 125.
Setting the value to anything other than 125 throws an exception.
In UTF-8 encoding, each Unicode code point (character) requires 1 to 4 bytes,
so the number of characters that can be stored may be less than the capacity.