System Types does not include DateTime

[FraserGreenroyd]

The list of system types here for the IsExtensionMethod check does not include DateTime.

It also only includes string and not String, and does not perform a ToLower() operation on the parameter meaning we are enforcing methods with String foo input to be this String foo which isn't ideal.

This should be updated to:

• Add DateTime to the excluded list
• ToLower() the parameter type so the list can accurately check it