Clean up string processing: delimiters

[cknitt]

First step in cleaning up string processing in the compiler. Define a unified String_kind.t as follows (replacing the existing delim type), make it mandatory and use it throughout the compiler pipeline (with the exception of the Parsetree for now).

    type t =
      | Standard (* Normal JS string with escaping *)
      | Verbatim (* Literal quoted string for tags *)
      | RawJs (* Raw JavaScript expressions *)
      | Template (* Template literals *)

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commit: 7379d01

[cristianoc]

@codex review this change and explain the use of the various kinds of strings. Is this just a refactor or could it change anything observable? Make comments about style, clarity and possible simplifications.

[cknitt]

@cristianoc For now it should be just a refactor that shouldn't change any behavior. I left out the Parsetree on purpose here as that's where things started breaking. 🙂 Can continue there later in a separate PR.