Implement SA0002: Invalid settings file

[sharwell]

It would be nice to use a compilation end event to report a single instance of this violation if we are unable to deserialize stylecop.json. This would catch (at least) the following situations:

1. JSON syntax errors (e.g. Analyzer doesn't read stylecop.json. #2028)
2. Semantic errors, such as using an array instead of a string or an unknown value for an enumerated type

This would not report a diagnostic if the file is not configured correctly.

[nbarbettini]

👍

Will probably be even more helpful in the future when/if the configuration becomes more complex.

[vweijsters]

👍