Add support for .tsx file extension (TypeScript-JSX)

[danjguzman]

When creating a new React App + TypeScript, the extensions of the files by default are .tsx.

When opening .tsx files in Phoenix, it defaults to Text which removes all highlighting. And sometimes (in my experience) it prevents the ability to change the file type to something else unless I close the app and try again.

TSX = Typescript-JSX which is not exactly the same as individual JSX or TypeScript types.

When switching to the nearest file type/syntax, Typescript, the highlighting is broken.

Switching to JSX seems like it might help, but it's the wrong type for .TSX.

Brackets had a 3rd party extension for TypeScript-JSX (TSX) type which is long outdated, but might help provide some context.

[abose]

Please track in #166