Add .BtnGroup-parent, deprecate .BtnGroup-form

[muan]

• There doesn't seem to be a dev branch for me to compare against
• For some reasons I can't push to primer repo directly anymore but I definitely still have admin access 🤔

Anyways this PR adds .BtnGroup-parent and keeps .BtnGroup-form for backwards compatibility. parent is a more generic name since this class can be used on more than just <form>. I had to use it on <details> a few times already.

Is this OK? is there more we need to do for class name deprecation?

🌹

[shawnbot]

I like this change a lot. It's something we noticed when attempting to re-implement the Merge pull request button from the merge box in primer-react, too.

This is definitely a breaking change, so we'd need to queue up removal for v11 in #498. In the meantime, we can call out the deprecation with @warn (as in colorizeTooltip() or .avatar-stack), update the style guide to reference the new class, then do a one-time search-and-replace to kill off any of the old instances.

[emplums]

This looks good to me! I agree with @shawnbot we should add a deprecation warning as part of this PR if possible! 💖

Regarding the dev branch, we've decided to get rid of it, and compare directly against our release branches instead. It's not in the documentation because we figured ds-core could be in charge of updating base branches, but in practice it feels a bit awkward 🤔 I'll think on that and update the docs accordingly!

[muan]

Does https://github.com/primer/primer/pull/522/files#diff-9a0d8f3df27d9a8ca7170fb44e64e75bR5 looks right?

[shawnbot]

Beautiful, thanks @muan! ❤️