Update grammar file for maintainability (#2270)

Consolidated duplicate code, added helper functions, and reviewed
the grammar file for issues.

NOTE: I used an AI tool to review and cleanup the grammar file. I
      have reviewed all of the work it did.

Improvements:

1. Added KEYWORD_STRDUP macro to eliminate hardcoded string lengths
2. Consolidated EXPLAIN statement handling into make_explain_stmt helper
3. Extracted WITH clause validation into validate_return_item_aliases helper
4. Created make_default_return_node helper for subquery return-less logic

Benefits:

- Reduced code duplication by ~150 lines
- Improved maintainability with helper functions
- Eliminated manual string length calculations (error-prone)

All 29 existing regression tests pass

modified:   src/backend/parser/cypher_gram.y

Author: jrgemignani