Validate literals at a later stage

[Vexu]

Most people would recognize 0x213g as a malformed hex literal and the same goes for '' being an empty char literal but currently Zig validates these kinds of error in the tokenizer, giving out terrible, vague errors like expected expression, found 'invalid bytes' and preventing formatting.

It would instead be better to have the tokenizer only validate things that actually prevent the tokens from being valid such as newlines in string literals (which should also give a proper error message instead of the same invalid bytes one) and leave everything else to AstGen.

[moosichu]

I'm keen to take a look at this once #12661 has been merged, but currently blocked on waiting for that or otherwise the work might potentially conflict there.