Investigate reducing raw-string lexing/parsing logic duplication.

[CyrusNajmabadi]

Currently, we have a bunch of logic duplicated in the lexer/parser for raw-strings. That's because normal raw-strings are 100% handled in the lexer. However, interpolated raw strings have to go through a lexer prepass (to find start/end/holes) then teh parser breaks taht all up and then does the raw-string logic handling. This means taht same logic in the lexer is then duplicated in the parser.

We could have normal-raw-strings work like interpolated-raw-strings. Lexer just finds the start/end, and then the parser takes over and does all the logic itself.

Came out of discussion in PR #59041

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