[3.x] Implement limited surrogate pairs support for platforms with 16-bit wchar_t.

[bruvzg]

Fixes #54617
Fixes #23093

Bugsquad edit: Fixes #52698

Adds support for serializing/deserializing from/to UTF-8 and displaying characters in the U+10000 - U+10FFFF on Windows.

No changes to the public scripting / add-on APIs.

Limitations:

• In the input controls, surrogates will act as separate characters (can be selected and deleted individually).
• All string functions also work with surrogates as separate characters.
• 3.x UTF-8 encoder/decoder can parse character above U+10FFFF, which are invalid Unicode characters and can't be encoded as surrogate, these characters are replaced with space as before.
• No kerning support for character encoded as surrogates.

Note: Text with the U+10000 - U+10FFFF characters in the existing projects created on Windows should be re-entered, since it was incorrectly encoded, existing projects created on other platforms should work on Windows without any changes.
It should be enough to open, save and reopen a scene with the incorrectly encoded surrogates on any platform to fix it.

[akien-mga]

Thanks!