Support multiple font faces in Web IDE

What does this MR do and why?

Support multiple font faces in Web IDE. In particular, this makes it render the proper italic version of GitLab Mono rather than a faux-italic one.

See gitlab-web-ide!242 (merged).

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the Web IDE, e.g., http://gdk.test:3000/-/ide/project/gitlab-org/gitlab-test/edit/master/-/
2. Open a file, e.g., README.md
3. Add some italic text, e.g., _foobar_
4. Observe the different italic glyphs, like f and a.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.