Improved tooling for React lesson

[egillespie]

Students who complete the React lesson often follow along with the React and JSX concepts well enough, but many ask questions about the tooling — both in VS Code and use of Babel in the browser.

The following improvements would address most of the questions:

1. Introduce VS Code extension(s) that lint JSX files (no "React is undefined", no errors about " characters in JSX, etc.)
2. Use a simple package.json setup to compile React into plain JavaScript before it reaches the browser