fix: Correct error message for invalid bytes in multiline strings

[WillLillis]

This change provides a correct error message when an invalid byte is found inside of a multiline string line. Taking the example from #20900:

const foo =
    \\const S = struct {
    \\<TAB>// hello
    \\}
;

Before:

src/main.zig:4:1: error: expected ';' after statement
    \\ // hello
^

After:

src/main.zig:4:5: error: expected 'a string literal', found invalid bytes
    \\ // hello
    ^~~~~~~~~~~
src/main.zig:4:7: note: invalid byte: '\t'
    \\ // hello
      ^~~~~~~~~~~

I'm not 100% confident in the set of invalid bytes I added in lowerAstErrors, as it's just based off of those listed in the multiline_string_literal_line case of Tokenizer.next(). I think this part of the PR in particular could benefit from a close second check. :)

I initially thought that the similar problem with invalid bytes inside comments (mentioned here), but there isn't a .comment variant (only .doc_comment and .container_doc_comment) of Token.Tag to report the expected token as. Another variant to Ast.Error.Tag could be added to handle this case, but I wanted to check before doing so.

One minor nit/ bikeshed question: The rendered error message for this case reads error: expected 'a string literal', found invalid bytes. Since this error is triggered by the first single invalid byte encountered by the tokenizer, should this message use "byte" instead?

Closes #20900