Infer a speculation rule's source from the other keys

[jeremyroman]

Fixes #169.

[github-actions]

Preview:

• speculation-rules.html (diff base)

[jeremyroman]

What do you think? First commit is my first instinct (if present validate it as now, just allow it to be absent). Second was your slight preference (ignore it entirely).

Honestly I think I have a slight preference for the former abstractly but don't feel super strongly (and admit it's nice not to have to test both).

This is pretty trivial and I'd hope we can use a lightweight process if this makes sense to you.

[jeremyroman]

One case that ignoring it would break (that our unit tests caught) -- a catch-all could previously be written {"source":"document"} with no conditions, but that's not possible if implicit "source" is the only option. At least not unless we make {} also universal, which feels excessively weird. Reverted the second commit but can discuss further.

[domenic]

I'm convinced, let's infer it!

Maybe also consider explainer updates in this PR?

[domenic]

Love it!!