Improve configuration

[rjmholt]

Configuration of PSSA2 should be easier.

In particular:

• Misconfigurations should present a useful error to users about expectation vs what was given
• Configuration should take a list of rule configurations and implicitly turn them into a dictionary, rather than require null dictionary entries
• Perhaps add more here

[bergmeister]

Have a think about how config should work from the editors POV as well. At the moment all individual configuration are exposed as individual settings to individually override the default settings. If we simplified it to a few JSON objects, we need to make sure that this is not the actual configuration but only the configuration overrides. This way the concept of the PSSA settings file would only be useful when using the cmdlets directly but editor users would only configure vs code settings.
How would user and workspace settings combine if it was a big object instead of just 1 value? VS-Code has the concept that workspace settings override user settings but in this case we don't want that on the whole object but only the individual properties