Add ESLint rule to ensure .ts extensions on imports

[Johennes]

When importing without an extension

import { MemoryCryptoStore } from "./crypto/store/memory-crypto-store";

this rule throws an error

/Users/jm/Code/matrix-js-sdk/src/matrix.ts
  19:35  error  require file extension '.ts'  node/file-extension-in-import

Sadly, it does not throw an error when the file is imported with a .js extension. I'm not sure why but I suppose forgetting the extension entirely is the most common case anyway.

Relates to: #4392

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[florianduros]

Thank you perfect