Specify environment in eslint

[filipsobol]

Currently, we do not specify in which environment we are running our code, so eslint does not know which globals we have to access. This forces us to use magic /* global XXX */ comments in our code whenever we want to use browser-specific globals like window, document, setTimeout, etc.

This is cumbersome and adds unnecessary bloat to our codebase.

I prepared two pull requests that add proper eslint configuration and remove unnecessary comments: #14806 and https://github.com/cksource/ckeditor5-commercial/pull/5452.

[filipsobol]

Concern raised by @niegowski in #14173 (comment):

Those comments could indicate where we use DOM APIs if we ever want to support headless editor mode (running without DOM on the node server). If we remove those, then it could be harder to spot. OTOH we could then switch to node env and check what is failing, but this would need to scan much more errors and could be overwhelming.