Add On-Demand Scan Command and Trigger On Settings Change

[gfs]

Is your feature request related to a problem? Please describe.
When using the VS Code extension of DevSkim if a user modifies a setting they still need to trigger a rescan of each open document in order to update the diagnostic findings.

Describe the solution you'd like
When the settings are changed, at least the currently open document should automatically be rescanned and diagnostics updated and likely each currently open document. It may also be prudent to have a command to manually scan all files in the workspace, even if they aren't currently open.

Describe alternatives you've considered
The current workaround is after modifying the settings the user can either make any change to a document to update its diagnostics (for example, a net no-op operation like adding then removing a space, or restart VS Code.

Additional context
See #659 for additional discussion.