Fix override parsing and preserve comments in JSX applications

[davesnx]

Summary

Follow-up to #16 and #17. Companion parser fix: ocaml-mlx/mlx#47.

Restore object-override expressions

The two-token GREATER RBRACE closer made ordinary override expressions ambiguous, including examples without a comparison:

{< x = true && false >}
{< x = if condition then 1 else 2 >}

Adding parentheses let these parse, but the formatter removed them and then failed its own syntax check. The previous workaround only handled top-level > comparisons.

• Emit a distinct GREATER_BEFORE_RBRACE token for the closing angle while leaving } to be lexed separately. The name makes clear that the token consumes only >, not the brace.
• Update both vendored parsers, including JSX and object-type closers; preserve whitespace before the brace.
• Remove the narrow parenthesis workaround now that normal override expressions parse again.

Preserve comments on hand-written JSX applications

Previously, this failed with BUG: comment changed because conversion to JSX omitted the commented unit argument:

(App.createElement ~children:xs (* keep *) ()) [@JSX]

• Retain the unit argument's location during JSX classification.
• Keep ordinary application syntax when the omitted unit has comments.
• Use the same classification for nested-child/prop parentheses, including spreads.
• Continue canonicalizing uncommented applications to <App>...xs</App>.

Tests

Added regressions for override comparisons, boolean operators, conditionals, local bindings, functions, match/try expressions, empty/local overrides, nested braces, object types, and JSX closing tags. Comment regressions cover unit-argument placement and applications nested in children, spreads, and props, including idempotency checks.

Passed the full local suite with OCaml 5.5.0 and Menhir 20230608:

DEVELOPER_DIR=/Library/Developer/CommandLineTools \
SDKROOT=/Library/Developer/CommandLineTools/SDKs/MacOSX15.4.sdk \
opam exec --switch=/Users/davesnx/Code/github/ocaml-mlx/mlx -- dune runtest

Publication note

The initial commit was accidentally pushed to main and reverted in 252fe2f. This PR reapplies the fixes through review and includes the explicit token name; main's content was restored before this branch was published.