Add TypeScript checks to actions

[ecxyzzy]

Currently our GitHub Actions tests don't include transpiling TypeScript to JavaScript, which is problematic because then no type checking is done on pull requests.

To make this happen, several minor things need to be done:

• Remove "watch": true from tsconfig.json. This stops the TypeScript compiler from running in "watch mode," which would cause the automated tests to time out.
• In the scripts object of package.json, add "build": "tsc".
• In .github/workflows/tests.yml, add - run: npm run build after - run: npm test.

[vinnyho]

I will take this one.