fix(postgresql): add jsontype rule and fix REVERSE keyword

Fix test failures by adding missing JsonType support and making REVERSE
available as a function name.

Changes:
1. Add jsontype rule matching PostgreSQL's gram.y:
   - jsontype: JSON
   - Added to simpletypename (for general type references)
   - Added to consttypename (for constant type references)

2. Add REVERSE to plsql_unreserved_keyword:
   - REVERSE is a PL/pgSQL token but should be usable as a function name
   - Now matches type_function_name via plsql_unreserved_keyword path

Root Cause Analysis:
- PostgreSQL's grammar has explicit JsonType rule for JSON keyword
- Our grammar was missing this, causing ::json typecast to fail
- JSON is a col_name_keyword, not in type_function_name categories
- Without jsontype rule, parser tried qualified_name%TYPE_P path

- REVERSE was in old builtin_function_name rule (now removed)
- After removal, REVERSE token couldn't be used as function name
- Solution: Add to plsql_unreserved_keyword to allow SQL usage

Test Results:
✅ All previously failing tests now pass:
   - json_encoding.sql
   - json.sql
   - jsonb.sql
   - join_hash.sql
   - builtin_functions_string.sql
   - text.sql
✅ Full test suite passes with no regressions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: h3n4l

postgresql/PostgreSQLParser.g4

@@ -3194,19 +3194,25 @@ simpletypename
    | character
    | constdatetime
    | constinterval (opt_interval? | OPEN_PAREN iconst CLOSE_PAREN)
+   | jsontype
    ;
 
 consttypename
    : numeric
    | constbit
    | constcharacter
    | constdatetime
+   | jsontype
    ;
 
 generictype
    : (type_function_name | LEFT | RIGHT) attrs? opt_type_modifiers?
    ;
 
+jsontype
+   : JSON
+   ;
+
 opt_type_modifiers
    : OPEN_PAREN expr_list CLOSE_PAREN
    ;
@@ -4835,6 +4841,7 @@ plsql_unreserved_keyword
    | RELATIVE_P
    | RESET
    | RETURN
+   | REVERSE
    //| RETURNED_SQLSTATE
    | ROLLBACK
    //| ROW_COUNT