add --bundle-dir option to web subcommand

[musjj]

Add an option for copying a packed web bundle directory to a user-specified directory. This lets you skip any error-prone parsing/guessing when locating the bundle directory, which is usually useful for CI-related tasks.

The behavior is modeled after the unstable --artifact-dir option from cargo:

• --artifact-dir (nee --out-dir) Tracking Issue rust-lang/cargo#6790

Fix: #649

[DaAlbrecht]

I'm open to this but do you think this is still needed even if the Bundle path is part of the API?

[DaAlbrecht]

My bad, wanted to just leave some comments, didn't mean to approve already

[BD103]

@musjj are you still willing to work on this PR? If not, I can label it so someone else can adopt the work! No pressure either way :)

[musjj]

Sorry for the incredibly long delay. But yeah, I'm still very much interested in this!

I think this feature is still helpful for reducing the guesswork that comes with copying bundle directories, even if target/web is stable API (you still need to infer the subdirectory name).

I've addressed the comments above.

[DaAlbrecht]

Thanks a lot and sorry for the delay! Could you also add a section to the Changelog, and then this is ready to go imo.

[musjj]

Ok, updated the changelog!

[cookie1170]

You should also mention it in the book in the 'Creating web bundles' section of 'Web Apps', otherwise looks good

[TimJentzsch]

Just one minor comment, looks good to me otherwise :)