Make the '@typescript-eslint/explicit-function-return-type' rule lint .ts/.tsx files only

[yangmingshan]

Current in a mixed JS/TS codebase (which is common), you will get unfixable lint errors reported within .js/.jsx files.

Documentation: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/explicit-function-return-type.md#configuring-in-a-mixed-jsts-codebase

[sindresorhus]

I think you should rather use overrides yourself and only extend this shareable config for TS files. It’s not meant for JS files.

[yangmingshan]

You have your point, I'm closing this PR.