XO should not lint type definition files in a non-TS project

[sindresorhus]

Many CommonJS (non-TS) projects have a TS type definition file, like index.d.ts and it currently has to be manually ignored because XO tries to lint it: https://travis-ci.org/sindresorhus/delay/jobs/418345973

How should we handle this? I don't think users should be forced to manually ignore type definition files.

We could ignore files ending in .d.ts unless the user has typescript-eslint-parser?

[pvdlg]

We could ignore files ending in .d.ts unless the user has typescript-eslint-parser?

Sounds a good idea.

[sindresorhus]

I decided to remove the extensions for now. They cause a lot of problems and I'm not sure what the correct solution is.

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We could ignore files ending in .d.ts unless the user has typescript-eslint-parser?

We actually cannot easily check whether the typescript parser is included as we would have to resolve the whole extends config chain, which we currently leave up to ESLint, and ESLint doesn't expose any API for us to do this.

[sindresorhus]

I think a better solution is this: eslint/eslint#10828 (Please vote on it)

[pvdlg]

@sindresorhus this can be closed since #426 was merged right?