Convert all API 'new FileSystem()' utilization to consume a static instance

[michaelcfanning]

We have patterns in code where API consult a parameter (which may be a test mock) and otherwise instantiate an instance of FileSystem() to perform work.

Rather than instantiating this class for each method call, we should have a singleton property (named Instance) that we use instead. The default file system capability is effectively a static helper class.

After this work, patterns like so:

fileSystem ??= new FileSystem();

will look like this:

fileSystem ??= FileSystem.Instance;

@eddynaka