StringUtil: Make UTF16ToUTF8 and UTF8ToUTF16 use custom encoding/decoding implementation.

[jordan-woyak]

Replaces: #14282.
Fixes: https://bugs.dolphin-emu.org/issues/13962

Note that std::wstring_convert throws exceptions on invalid sequences, and it's deprecated, to be removed in C++26.

This PR replaces usage of wstring_convert with hand-rolled UTF8/16 encoding/decoding.

I'd usually rather just use an existing libarary (like iconv) but..

• The iconv dependency isn't set up in the Windows build.
• WinAPI's MultiByteToWideChar can't output to a std::u16string without strict aliasing violations or a memcpy.
• UTF8/16 really isn't overly complicated.
• These hand-rolled versions seem to be 5-10x faster than iconv in my simple tests.
• We have more control over how semi-invalid sequences are handled. This issue will be fixed if QString conversion functions are changed to use the custom decoder.

If we wanted to entirely eliminate the iconv dependency, CP1252 would be easy enough to implement, but ShiftJIS looks like it isn't so fun.
Edit: I have these implemented locally if we want to do that. ShiftJIS wasn't so bad.. just some giant tables.