Support index.jsx entry file

While I understand that the jsx extension is supported but not recommended, I was surprised that my index.jsx file isn't being recognized after renaming it to pass eslint-config-airbnb's jsx extension requirement.

Is this a bug report?

Sort of, I was surprised this wasn't supported but it may be an intentionally missing feature.

Which terms did you search for in User Guide?

index.js, it's clear that it's currently expected for the filename to be an exact match.

Steps to Reproduce

1. Rename index.js to index.jsx
2. npm start

Expected Behavior

App runs normally, treating index.jsx as the same as index.js when it's not present.

Actual Behavior

> react-scripts start

Could not find a required file.
  Name: index.js
  Searched in: /Users/nick/Repos/password-entropy/src
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! @ start: `react-scripts start`
npm ERR! Exit status 1
npm ERR! 
npm ERR! Failed at the @ start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.