Request the user JS source code and source maps from Firefox

[canova]

We would like to be able to get the source maps and show the JS source in the source code view eventually. To be able to do that, we have a way to request these from Firefox via webchannel. This requires changes on both the frontend and backend. This issue is for adding this webchannel support for the frontend.

Backend bug can be found here: https://bugzilla.mozilla.org/show_bug.cgi?id=1916785

Some notes from the past meeting:

• We can reuse some of the devtools code to get source mapped and prettified version of a bundle.
• First step: request it on profile load, but do not store it.
  • Question: store a hash that we could check when fetching it?
• Later:
  • Drag and drop a source map.
  • Support WebAssembly files.

This is one of the tasks that belong to #3741.