feat: add fixture pipeline using @libpg-query/parser for plpgsql-deparser

- Add @libpg-query/parser as dev dependency for parsing PL/pgSQL
- Create test-utils with fixture pipeline that loads from __fixtures__/plpgsql/
- Add jest.config.js for TypeScript support in tests
- Update tests to use real parser output instead of hardcoded JSON fixtures
- Tests now demonstrate proven pipeline: SQL fixtures -> parser -> deparser

Author: pyramation