Add notification support for long running coverage operations

## Info
- implement notifications to allow for visibility into extension operation status
- implement cancellation for long running operations
  - coverage searching
  - coverage file loading
  - etc

![notifications](https://code.visualstudio.com/assets/updates/1_22/long-running.gif)

## Reference
https://code.visualstudio.com/updates/v1_22#_extension-authoring