Replace Prefire with a very similar but simpler and more direct approach (#3232)

* Replace Prefire with a very similar but simpler and more direct approach

* Move PreviewTest snapshots to their new location (snapshotting default, mirrors UITests)

* Update acknowledgements

* Switch snapshot configuration extensions to a custom version

* Exclude snapshots from the PreviewTests target

* Fix acknowledgements

* Rename snapshotting preference keys

* Remove `skipPackagePluginValidation`

PreviewTests/SupportingFiles/target.yml

@@ -29,10 +29,6 @@ targets:
     type: bundle.unit-test
     platform: iOS
     
-    buildToolPlugins:
-    - plugin: PrefireTestsPlugin
-      package: Prefire
-
     dependencies:
     - target: ElementX
     - package: SnapshotTesting
@@ -48,4 +44,7 @@ targets:
       release:
 
     sources:
+    - path: ../Sources
+      excludes:
+        - "**/__Snapshots__/**"
     - path: ../SupportingFiles