v0.3.0

feat: add 7 new linter rules and expand documentation

- DELETE without WHERE warning
- SELECT * warning
- = NULL error (use IS NULL)
- IN () empty list error
- LIMIT without ORDER BY warning
- WHERE 1=1 always-true warning
- Subquery without alias warning
- Linter tests moved to tests/linter_tests.rs
- README: added Syntax Checking section and full linter docs