Support '.tsx' filename extension by default

[kripod]

As TypeScript is spreading and official ESLint support is given through the typescript-eslint project, I thought it would be convenient if the .tsx file extension was supported out of the box.

[ljharb]

Hmm. I can see how this would help a TS user, but I'm not sure it's a sensible default - a non-TS user would actively want tsx files to be prohibited.

[kripod]

@ljharb I don't think that a non-TS user is willing to create or transpile tsx files at all. We cannot prevent them from creating a file of a given kind anyway.

[ljharb]

I agree with you - that's why i'm saying that the linter rule should error on a tsx file for a non-TSX user.