Changes to address IFileSystem.Watch method addition

Absorbs the new IFileSystem interface. This change is to just address the breaking change introduced in IFileSystem.
Razor has to do the necessary changes to subscribe to the Watch event for expiring the modified files.
Remove use of `Assert.DoesNotThrow()`
- no longer in xunit :smiley:
Removing transitive references from aspnetcore50 to unblock build
Reacting to System.Xml.XmlSerializer version change
Update tests for Razor Tag Helpers Unique ID feature:
- aspnet/razor#241
Merge branch 'release' into dev
added few taghelper functional tests
removed the use of db