Add unit tests, integration tests, and CI

[deepa0128]

What

Wires up a full test suite and GitHub Actions CI pipeline.

Unit tests — 103 tests, no database required

File	What it covers
test/unit/identifier.test.ts	Identifier safety — valid names, all rejection cases
test/unit/builder/expr.test.ts	All 11 expression helpers
test/unit/builder/select.test.ts	SelectBuilder — columns, where, orderBy, limit, offset, error cases
test/unit/builder/insert.test.ts	InsertBuilder — single/multi-row, error cases
test/unit/builder/update.test.ts	UpdateBuilder — sets, where, no-where allowed
test/unit/builder/delete.test.ts	DeleteBuilder — where, no-where allowed
test/unit/dialect/compileQuery.test.ts	Full SQL compiler — exact SQL strings and params for both dialects, all operators, edge cases

Integration tests — skipped locally without a database

Tests hit real Postgres and MySQL instances. Skipped automatically when POSTGRES_URL / MYSQL_URL env vars are absent, so npm test always passes locally without Docker.

Covers: INSERT, SELECT (with filters / limit / offset), UPDATE, DELETE, transaction commit, transaction rollback, nested-transaction rejection, and ConstraintError on unique violations.

CI (.github/workflows/ci.yml)

Runs on every push and PR across Node 18 / 20 / 22. Spins up Postgres 16 and MySQL 8 service containers with health checks, then runs:

1. tsc --noEmit on source
2. tsc --noEmit on tests (tsconfig.test.json)
3. npm run build
4. npm test (unit)
5. npm run test:integration (with POSTGRES_URL / MYSQL_URL injected)

Test scripts added to package.json

npm test                  # unit tests only
npm run test:watch        # unit tests in watch mode
npm run test:integration  # integration tests (needs DB)
npm run test:all          # both
npm run typecheck         # tsc on src + test together

Stacks on #3.