Configure eslint to lint ts files

[NotWoods]

I think ESLint only checks JS files by default. Adjusts the command to check ts files and adds the typescript parser. @typescript-eslint/eslint-plugin could be added for TS-specific rules.

[PEConn]

Thanks for this! Will eslint with the TypeScript plugin do the same as TSLint or are there differences?

@andreban this LGTM, over to you.

[andreban]

LGTM

@NotWoods Thanks for this! We should definitely add @typescript-eslint/eslint-plugin. Let me know if you want to add it to this PR, send in another one, or I can pick it up and make the PR.

[NotWoods]

I can add the plugin in 🙂