Add bazel_features.bzlmod.is_enabled

[fmeum]

No description provided.

[fmeum]

This is mostly me exploring the design space of this repo - this could be a useful addition if we think it's the right place for it.

[Wyverald]

I'm a bit undecided. Not entirely sure we want this sort of "flag detection" stuff. Any pros and cons in mind?

[brentleyjones]

Pros:

• People ask for this a lot
• There isn't a global attribute to look at to see if it's enabled
• That's what this features repo is about, right?

Cons:

• I can't really think of any? If people need this (and there are numerous examples of the need), why not add it?

[Wyverald]

I'm sufficiently convinced -- tried thinking of substantial cons, couldn't think of any.