Add Audio Unit and VST plugin folder artifiacts

[joshka]

This is an updated version of the PR for this feature that works properly

[adidalal]

@jawshooah This looks ready to merge.

cask_language_deltas.md will also need to be updated, but as you said, incrementing the version doesn't do anything at this point, so it's up to you.

[jawshooah]

Waiting on #15381 (which is in turn waiting on Homebrew/legacy-homebrew#46795) so we don't have to push a new release for this to work.

[adidalal]

@joshka As the previously blocking PR is now merged, would you mind adding the two stanzas to https://github.com/caskroom/homebrew-cask/blob/master/doc/cask_language_deltas.md (under the All Supported Stanzas (1.0) section)?

That way we can keep this PR self-contained and also go ahead and merge it.

[joshka]

Sure - will do so a little later tonight

[adidalal]

@jawshooah Do we need cask_language_deltas anymore?

Updating that seems to be the only blocker for this merge, (and I don't mind adding it in if does need to be updated - it's like a two line fix)

[joshka]

I got busy and forgot to get around to this. My biggest hassle was understanding how you're versioning the DSL. Happy for you to update this @adityadalal924

[jawshooah]

@adityadalal924 Please do go ahead and add those lines after merging. We likely don't need cask_language_deltas anymore, but I'd like for it to remain up-to-date until we remove it, along with any code relating to DSL versioning.

[adidalal]

@joshka Will merge and update documentation accordingly. I'll put a message over in #15644 once everything is done so you can update that PR accordingly as well.