[release/6.0] Initial Bootstrap v4 SxS support (#36378)

* Add props and build files

* Bring back V4 files

tmp

Setup V5 stuff

* Add back v5 files too

Restore V4 files

Undo diffs

Update Testing.DefaultWebSite.StaticWebAssets.V5.xml

Fix tests

Checkpoint

Fix

Undo

Try bootstrap4 codepath

Stuff

Checkpoint before rebase

* Correct target names

* Fix views

* Cleanup

* Fix test

* Bring back targets

* Make things work

* PR feedback

* Fix for content type

* Harden the view version logic

* Fix test

* Nullable warning fixes

* Fix nullable warning another way

* Fix V4 nullability warnings

* Another nullability fix

* Fix CI

Co-authored-by: Javier Calvarro Nelson <jacalvar@microsoft.com>
Co-authored-by: Hao Kung <HaoK@users.noreply.github.com>