Support syntax highlighting for Flow enums

[gkz]

Adds support for highlighting Flow enums. For some background on the syntax, you can see https://github.com/gkz/enums

Flow Parser, Babel parser (with Flow syntax plugin), Prettier, and Babel-ESLint already support Flow Enums.

This is the first time I have edited TextMate grammars. I have attempted to follow the existing patterns, but feel free to point out if I am doing something incorrectly.

One (convenient) thing to note is that enum is already a reserved keyword in JS.

[michaelgmcd]

Thanks for the PR! I’m hoping to review and test it this week.

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[gkz]

Thank you! Just FYI I plan some updates to the PR later today