Reserve guarded string literals (RFC 3593)

[pitaj]

Implementation for RFC 3593, including:

• lexer / parser changes
• diagnostics
• migration lint
• tests

We reserve #", ##", ###", ####, and any other string of four or more repeated #. This avoids infinite lookahead in the lexer, though we still use infinite lookahead in the parser to provide better forward compatibility diagnostics.

This PR does not implement any special lexing of the string internals:

• strings preceded by one or more # are denied
• regardless of the number of trailing #
• string contents are lexed as if it was just a bare "string"

Tracking issue: #123735
RFC: rust-lang/rfcs#3593