feat: custom label beta 2

[mshanemc]

What does this PR do?

after beta feedback forcedotcom/cli#2840 and discussion forcedotcom/cli#2948, update the way labels decompose.

Meta

• CustomLabels and CustomLabel both use the default Adapter (no change) and can stand alone as SourceComponents. There is no relationship between them, except for the output of the Transformers
• CustomLabel is always used for source format (1 file per label)
• CustomLabels is always uses for metadata format (1 file with all the labels in it. named CustomLabels)
• since the mergeSet will be a bunch of customLabel, but the retrieve is CustomLabels, toSourceFormat needs the entire mergeSet (and not a single mergeWith SourceComponent) to find components with the matching fullName (for labels outside the default dir, to avoid creating duplicates).

change the decomposeCustomLabelsBeta to do one file per label (preset update)

• 2 new transformer classes in file to handle label/labels
• new finalizer for labels
• the toSourceFormat method now takes an object (component, mergeComponent, and [new] the entire mergeSet)
• presets can put an empty string "" for suffix/childTypes/strictDirs to remove an entry from the default registry (this preset removes the parent/child relationship between label/labels)

other

• TODOs for removing exported Transformers in the next major. These aren't used that I could see and should never have been exported.

QA notes

there are some small changes in STL necessary to make this behave right forcedotcom/source-tracking#651 and you'll need those for testing tracking.

Tested during development

• local create, preview, deploy
• deploy via mpd
• remote update, track, retrieve
• remote delete, track, retrieve
• local delete, track, retrieve
• retrieve into mpd (existing local labels with remote change update, new labels are placed in default dir, can do both)
• label file outside of the /labels folder in source format (deploy, retrieve using -m CustomLabel:foo)
• subfolders within the /labels directory for grouping labels (deploy, retrieve changes)
• retrieve using --metadata CustomLabels ("give me all of them") doesn't match the local CustomLabel and put them in the right places (they'll all end up in the default directory). Workaround --metadata CustomLabel (using --metadata CustomLabels now throws an error when the new Transformer is used).

What issues does this PR fix or reference?

@W-16375268@