Fix some webpack warnings

[msujew]

What it does

Related to #11432

This change fixes/circumvents 3 kinds of webpack warnings:

1. Missing source maps. We ignore those, as these come from dependencies and it doesn't really harm us too much
2. Weird require usage in monaco on a path which is never executed. We can ignore that as well
3. Unable to inline url('') css. I've replaced the URL with a gradient which serves the same result

How to test

Only the third point can actually be tested. It's a regression test against #7751. The plugin icons should be empty instead of a grey box.

For everything else: The related webpack warnings shouldn't appear anymore when running theia build.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes look good to me 👍

• confirmed that source map warnings are no longer present
• custom icons from plugins do not display the grey square on reload