feat: raw identifier syntax

[davesnx]

Add support for raw identifiers (OCaml 5.2 / RFC 27) to the MLX lexer, allowing keywords to be used as identifiers via the # prefix.

This unblocks polyvariant tags that collide with OCaml keywords, which is common for HTML attribute values:

`\#lazy    (* loading="lazy" *)
`\#true    (* aria-pressed="true" *)
`\#false   (* aria-pressed="false" *)
`\#lazy    (* loading="lazy" *)`\#true    (* aria-pressed="true" *)`\#false   (* aria-pressed="false" *)

The implementation mirrors the OCaml compiler's lexer: a raw_ident_escape is matched before the regular identifier rules, emitting LIDENT directly without keyword-table lookup. Applied to LABEL, OPTLABEL, LIDENT, and JSX_LIDENT rules across mlx/lexer.mll and ocamlmerlin_mlx/ocaml/preprocess/lexer_raw.mll.

It also allows <#lazy> as a component name

[andreypopp]

I wonder if we should exercise updating to 5.4 AST instead? This will get us this and other features like named tuples for free.

[davesnx]

Totally made the update here #29