feat(expander): port expander to use internal AST types

Move expander from internal/engine/postgresql/expander to internal/x/expander
and port it to use the internal AST types instead of pg_query nodes.

Key changes:
- Use internal AST types (*ast.SelectStmt, *ast.InsertStmt, etc.)
- Use astutils.Search for star detection
- Use ast.Format instead of pg_query deparse
- Add Parser interface for dependency injection
- Add test cases for COUNT(*) (should not be expanded)

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kyleconroy