Make it easier to unit test the code base

[se02035]

Problem

The current code base is difficult to unit test; in most of the cases integration tests are required (these tests are typically slower and some elements are more complicated to mock/automate). Unit testing extensions is currently difficult. see microsoft/vscode#94746

Solution

Restructure the code base and use standard recommended practices (such as decoupling/loose coupling of components, integration/abstraction layer) to improve testability and maintainability.

There is a thin integration layer encapsulating the vscode module. The extension code communicates with the vscode module only via this layer.