Evaluate `ecmaFeatures` option

[kaicataldo]

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What version of typescript-estree are you using?
master

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Had a thought while I was looking at eslint/typescript-eslint-parser#530. Now that the ESLint-compatibility layer has been separated out, does it make sense to keep the ecmaFeatures option? It looks as though it's only used to enable JSX.

I'm not entirely sure ecmaFeatures makes sense for this package - perhaps we could just add either a jsx: true or enableJSX: true option?

We could always make this backwards compatible, if that's a worry. I'm more than happy to make a PR!

[JamesHenry]

Thanks again for doing this