Add raw string syntax {|...|} and {id|...|id}

[toots]

Summary

Adds a raw string literal syntax that suppresses both escape processing and #{} interpolation. Content is taken verbatim from source.

The syntax is inspired by OCaml's quoted string literals ({id|...|id}),

This PR implements the changes required to support the feature requested in #4957.

Syntax

• {|...|} — basic form, terminates at the first |}
• {id|...|id} — delimited form, where id is any sequence of lowercase letters and underscores; terminates only at |id} with the matching id

This allows embedding |} inside a raw string by choosing a non-empty id:

s = {|hello #{world}|}          # => "hello #{world}"
s = {foo|contains |} here|foo}  # => "contains |} here"
s = {||}                        # => ""

Implementation

The lexer matches {[a-z_]*| as the opening delimiter, extracts the id, and delegates to read_raw_string. That function scans until it finds |[a-z_]*} with a matching id, treating any non-matching |..} sequences as literal content.

A new RAW_STRING token carries the raw content string. The pipeline adds:

• `Raw_string of (string * string) variant in parsed_ast
• passthrough in term_preprocessor (no expand_string processing)
• reduction `Raw_string (_, s) -> `String s in term_reducer
• JSON serialization in parsed_json with both id separator and string content.

[github-actions]

Scheduled for backport to v2.4.x-latest when this PR is merged.