Allow a 1:1 relationship between UserScript and a script context for a frame

[Chris Coulson]

Currently there is a 1:1 relationship between UserScript and a file on disk. If the embedder wants to load more than 1 file in to a script context, it creates multiple UserScript objects with the same context.

At the moment, we have nowhere to customize settings for a script context in which user scripts execute. Things we might want to customize are:
- Should the context be debuggable in devtools?
- CSP settings
- Security origin

These would make sense on UserScript, if UserScript had a 1:1 relationship with a script context within a frame. I think we update UserScript to allow this.

What I propose is:

- Deprecate creating more than 1 UserScript with the same context (allow it for now, but print a warning if an application does that).
- Provide a new property to allow the embedder to specify multiple script URLs.
- Provide APIs to control when / where the scripts are injected (equivalent to greasemonkey's include, exclude, run-at), and ignore inline greasemonkey metadata for multiple-source UserScripts.
- UserScripts that inject a single source (using UserScript.url) will still be able to be controlled using inline greasemonkey metadata