Provide an easy way of displaying prettified JavaScript source code for a given function

This is part of #820, but JS-specific.

For JS slowness, the stacks in a profile are usually not enough information to determine whether or why something is a problem. When JS engine developers identify a potentially problematic JS function, they usually need to know what the JS function is doing, and they go through this manual process:

* Open the script URL for the function, copy the source code into an editor
* Identify the right function based on line and column number
* Copy and paste this JS function into https://beautifier.io/
* Read the beautified source

It would be really nice if we could automate this process, even before we have source map support.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-69)