Allow watchers for the active project

[timholy]

This allows packages (with Revise as the intended target) to register
themselves to be notified when the active project changes.
This will allow Revise to keep track of the appropriate source for
non-default environments.

For practical effect, Pkg (and perhaps packages like Pluto) will have
to switch from setting Base.ACTIVE_PROJECT[] directly to calling
Base.set_active_project.

[timholy]

I'll merge soon, barring further concerns.

[DilumAluthge]

For practical effect, Pkg (and perhaps packages like Pluto) will have
to switch from setting Base.ACTIVE_PROJECT[] directly to calling
Base.set_active_project.

@timholy After this PR is merged, could you make a PR to Pkg.jl with the necessary changes?

[timholy]

Done. The test will be noisy until #41625 is fixed.

[timholy]

I don't think those are due to this PR, but it's a lot of failures so let's just see if things have improved.

[DilumAluthge]

@timholy Can you rebase on the latest master?

[DilumAluthge]

@timholy If I understand correctly, currently to use this feature, you provide a callback function f() that takes no arguments, right?

Would it instead make sense for the user to pass a callback function f(x)? And then when we call the callback function, we pass the new active project as the positional argument?

So then, users don't need to make an extra call to Base.active_project() inside their callback function?

[timholy]

With the possibility of @async callback processing, I thought it might be better to have the user call it directly when ready to handle the change (since of course it could update again between the time of making the callback and when the @async task runs). But this is probably biased by Revise's use case.