Better support multiline strings (herdocs) and string tagging

[MatheusRich]

Lit should support multiline string blocks for embedding SQL, JSON, Markdown, regexes, etc. A block can optionally be labeled (@sql, @json, etc.) so editors can apply syntax highlighting, but the label is not required.

Syntax:

[@<label>]
<marker><content>
<marker><content>
...

• The first content line determines the mode:

  • ''' → raw (no escapes, no interpolation)
  • """ → interpolated (supports escapes and #{…})

• Every body line must start with the same marker.

• Everything after the marker is included verbatim (spaces preserved).

• A line that is just the marker produces a blank line (\n).

• To include a literal leading quote, add one extra quote ('''' or """").

• The block ends at the first line that doesn’t start with the marker.

Examples

Raw SQL:

@sql
'''SELECT *
'''FROM users
'''WHERE id = 42

Interpolated SQL:

@sql
"""SELECT *
"""FROM users
"""WHERE id = #{user_id}

JSON (no label):

'''{
'''  "ok": true,
'''  "id": 42
'''}

Single-line form:

let query = @sql """SELECT * FROM users WHERE id = #{id}
let json  = '''{"ok": true, "id": 42}

---

This design keeps the marker visible on every line (like Zig) while @label provides a clean way for editors to apply language-aware highlighting.