Enable unit test code coverage

[andyleejordan]

Prerequisites

• [X] I have written a descriptive issue title.
• [X] I have searched all issues to ensure it has not already been reported.

Summary

• [x] Probably using Istanbul AKA nyc.
• [x] It should be integrated into the pipeline,
• [ ] significant decreases in coverage should be a failing PR status check,
• [ ] and a badge should be added to the readme.

Note this is specifically for the extension's tests, not Editor Services (which should also have it enabled).

Proposed Design

No response