Add components to support skeleton loading states (#4052)

* adds skeleton components and stories

* supports use of skeletons in AvatarStack and AvatarPair

* replaces ad-hoc skeleton loaders in DataTable and TreeView

* adds VRTs

* rm unused stuff from stories

* adds component prop docs for Skeleton components

* adds changeset

* disable animations for VRTs and aXe tests

* fix typos in skeletonbone VRTs/AATs

* test(vrt): update snapshots

* adds skeleton loader example using React Suspense

* renames 'SkeletonBone' to 'SkeletonBox'

* updates needed since this PR was opened so long ago

* try resetting package-lock to whats in main

* appease the linter

* moves BaseSkeletonBox styles to SkeletonBox, and co-locates skeleton component specific styles to their components

* replace 'sx' prop with 'className' where possible

* rm sx usage, appease linter

* updates snaps, rms sx from story argtypes

* test(Skeleton): update id for SkeletonBox stories

* chore: update snapshots

* test(vrt): update snapshots

---------

Co-authored-by: mperrotti <mperrotti@users.noreply.github.com>
Co-authored-by: Josh Black <joshblack@users.noreply.github.com>

.changeset/chilled-dolphins-eat.md

@@ -0,0 +1,7 @@
+---
+'@primer/react': minor
+---
+
+Adds components to support skeleton loading states, and uses those components to replace ad-hoc skeleton loading states in Primer React components.
+
+<!-- Changed components: SkeletonAvatar, SkeletonBox, SkeletonText -->