🎨 add first class debugging support

Created by: cdaringe

continuation of #1360. re-implemented per code review remarks, and conflicts resolved. needs 👀 re-review :).

problem statement

• react-scripts does not offer ready-to-roll jest debugging.

solution

• add it!

closes #594 (closed)

test

• checkout this feature branch
• /path/to/this/branch/create-react-app/bin/create-react-app dummy-app
• cd dummy-app
• modify npm test to add a debug flag ^^
• tell your editor to honor source maps
• npm test
• connect the remote debugger
• go.