Add vitest test framework

Adds two new npm scripts:

- 'test' - runs test suite once and exits
- 'test:watch' - runs test suite perpetually.

Any file suffixed with '.test.[js|ts]' is considered a test suite

.github/workflows/tsc.yml

@@ -27,3 +27,6 @@ jobs:
 
       - name: Run tsc
         run: npm run build:check
+
+      - name: Run test suite
+        run: npm run test