Returns the first UTF-8 character that cannot be converted to UCS-2

[bmickael]

Hello, I am submitting this merge request because I find it more practical to know which UTF-8 character could not be converted in the event of an error.

I tried to adhere as closely as possible to the crate's philosophy; #nostd and const fn aren't areas I work with on a daily basis.

[phip1611]

thanks, will take a look soon! by the way, your commiter email <toto@toto21.com> seems not to be linked with your github account - intentional? @bmickael

[phip1611]

Multiple things are going on here, this should be multiple commits. Also many changes are missing the corresponding motivation.

I asked Claude for assistance here and it sais: the misplaced paren in offset + 2 & 0x3f masks the index instead of the byte, which silently corrupts decoding for any 4-byte char at offset ≥ 62 and can hit unreachable_unchecked (UB), so it needs that fix plus a non-zero-offset test before merging.