Request source code for the Firefox-internal JS code

[canova]

For the source code view, we should have a way to request the source code for the Firefox-internal JS code (chrome:// etc.).

They can be hosted on hg.mozilla.org, or for local builds, they can be found locally.

Question: How do we get the content for a resource:// or chrome:// file?

• Local builds: access resources directly or through the Web Channel
• Nightly / Beta / Release resources: need a mapping from resource:// and chrome:// to the source file

Backend side of this task: https://bugzilla.mozilla.org/show_bug.cgi?id=1916789

This is a part of #3741 as well.

┆Issue is synchronized with this Jira Task