added JSX formatting to JavaScript

[lukkea]

Description

This comment by @hbergren prompted me to pull this repo into syntax-highlighting. With a couple of tweeks it's working.

I have added a "JSX" option to the menu. I also wondered if there should also be a "React" option that does the same as this, but that seemed unnecessary to me so I didn't do that.

Checklist:

Please replace the space inside the brackets with an x and fill out the ellipses if the following items apply:

• I've read and understood the contribution guidelines
• I've tested my changes against at least one of the following Anki builds:
  • Latest standard Anki 2.1 binary build [required for Anki-compatible 2.1 add-ons]
  • Latest alternative Anki 2.1 binary build
  • Latest Anki 2.0 binary build [required for Anki 2.0-compatible add-ons]
• I've tested my changes on at least one of the following platforms:
  • Linux, version:
  • Windows, version:
  • macOS, version:
• My changes potentially affect non-desktop platforms, of which I've tested:
  • AnkiMobile, version:
  • AnkiDroid, version:
  • AnkiWeb