core: Refactor XML unescape and error handling

[SuchAFuriousDeath]

While working on #21107, I discovered that quick_xml errors do not map 1:1 with Flash errors, so make_xml_error needs to be refactored (and all the code adapted). Doing it in that PR would be too hectic (quick-xml has had major breaking changes), so I'm submitting this separately. This PR also does some minor refactors and implements a FIXME.

No functional changes intended, everything should map to the exact same error code as before. There's quite a lot of xml tests, so if they all pass, we can be reasonably sure this does not break anything.

Context for the decoder usage removal:

The decoder doesn't actually do anything special for us, since we don't use the encoding feature. So there's no need to pass it around.

https://docs.rs/quick-xml/latest/quick_xml/encoding/struct.Decoder.html

If feature encoding is disabled, the decoder is always UTF-8 decoder: any XML declarations are ignored.

It can be seen here that without the encoding feature, std::str::from_utf8 is all the decoder does.
https://docs.rs/quick-xml/latest/src/quick_xml/encoding.rs.html#121-129