Add an upstream JSON fragment lexer

Summary: Ref T9566. This seems to be >100x faster than an older version of pygmentize for JSON inputs with a single long line. See discussion in D14273.

Test Plan:

• Verified runtime improvement by comparing scripts/highlight.php on the one-line.json test case from D14273 with the new lexer enabled and commented out. Saw >10x local improvement. My local base runtime is also >10x better than the runtime from D14273. My laptop might be a little faster than whatever that hardware was, but this is probably mostly pygmentize improvements in my most recent version.
• Added a test case.
• Used the highlighter to produce reasonable-looking results:

Reviewers: chad, gd

Reviewed By: chad, gd

Maniphest Tasks: T9566

Differential Revision: https://secure.phabricator.com/D14292