Use new monospace font in Monaco Editor

What does this MR do and why?

It updates the default font for the Monaco editor.

How to set up and validate locally

Make sure that the :vscode_web_ide feature flag is NOT enabled. Updating fonts in the new Web IDE is out of scope here.

Follow the instructions from this MR to apply new fonts. Go to Project => Repository => Try editing the file in Web IDE or edit a single file. Inspect the text in the editor and make sure that JetBrains Mono font is used.

References #382716 (closed)