vscode: extension activation scope

[flyinghermit]

To access files and directories, extensions should register activation points such as when a file is a python file, if the workspace is under a certain directory, or when the user manually activates the extension, etc. Activating our extension when a python file is opened in the editor is the easiest way to implement it with zero user effort. But we also don't want our extension to be activated when users are working with other python projects.

To scope down the extension activation point, we have two options:

1. Implement "activate" command that allows users to manually activate/deactivate predicate extension.
2. Auto-activate on certain python files and directories. For example, activating when filenames are <policy_name>.predicate.py or activating under a directory named predicate_policies.