Add a lint rule to disallow .ts files, enforce .tsx

[kumar303]

We want to always create .tsx files, not .ts files. This reduces the mental burden of trying to figure out which file to create. This lint rule is important because our lint command in #74 will ignore .ts files.

[willdurand]

Could be done with https://www.npmjs.com/package/eslint-plugin-filenames actually nope