Fix: allow reserved words as property identifiers after optional chaining

[rbonestell]

Summary

Fixes #377.

Reserved words (delete, class, return, etc.) are correctly treated as property_identifier nodes after . but produce ERROR nodes after ?. (optional chaining). This is because the reserved('properties', ...) context is not propagated through the ?. token path — likely a tree-sitter core issue.

This PR works around it by explicitly listing all reserved words as valid alternatives in the member_expression property field via a new _keyword_identifier rule.

Before:

a?.delete(b);  →  ERROR
a?.class;      →  ERROR
a?.return;     →  ERROR

After:

a?.delete(b);  →  (call_expression (member_expression … (optional_chain) (property_identifier)) (arguments …))
a?.class;      →  (member_expression … (optional_chain) (property_identifier))
a?.return;     →  (member_expression … (optional_chain) (property_identifier))

Changes

• grammar.js: Extract RESERVED_WORDS and CONTEXTUAL_KEYWORDS as shared constants to eliminate duplication across reserved.global, _reserved_identifier, and the new _keyword_identifier rule. Add _keyword_identifier (inlined) to the member_expression property field alongside $.identifier inside the existing reserved('properties', ...) wrapper.
• test/corpus/expressions.txt: Add test case for optional chaining with reserved words.
• package.json: Add tree-sitter to devDependencies (required by bindings/node/binding_test.js).
• .github/workflows/ci.yml: Pin node-version: 22 for Node binding tests (tree-sitter@0.25.0 cannot compile with Node 24).
• src/: Regenerated via tree-sitter generate.

Notes

• Parser impact is minimal: +10 states (+0.5%), +2.9% file size. LARGE_STATE_COUNT unchanged.
• The reserved('properties', ...) wrapper is retained for the . path and forward compatibility — when tree-sitter core fixes the propagation issue, the explicit alternatives become redundant but harmless.
• No existing tests are affected. All 117 corpus tests pass on all platforms.