Add .ts and .tsx support

[brendanoffer]

Add TS and TSX support to ESLint.

Typescript linting has been done in the past by TSLint, which is now being deprecated. The TSLint team is pointing users to use @typescript-eslint for typescript support; the Microsoft Typescript team, themselves, have been transferring their linting capabilities to use ESLint.

If users do not want their typescript files to be linted support they can simply add *.ts and *.tsx to their .eslintignore file.

[hound]

Freeze mutable objects assigned to constants.

[gylaz]

Thanks @brendanoffer. Would you mind fixing the Hound violation?

Additionally, I'm wondering if we need a specific guard to avoid unexpected linting of TS files for our current users. Those users are most likely using TSLint. If they also have ESLint enabled for some reason, will they see similar comments from 2 linters?

[brendanoffer]

@gylaz I can add freeze here if you like though I am not sure it is the best approach. I don't see any other regex's in the hound codebase being frozen.

Here is a Rubocop thread about freezing non-strings
rubocop/rubocop#6331

[brendanoffer]

@gylaz any guidance?

[gylaz]

Thanks @brendanoffer! Merging this as-is. We'll see if we get any feedback about potentially linting TS files with both ESLint and TSLint (and how often that arises).