Enable unit test code coverage

[andyleejordan]

Prerequisites

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

Summary

• Probably using Istanbul AKA nyc.
• 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

[andyleejordan]

So, sad news. The only tool I can find in the JavaScript/TypeScript/Mocha universe is Istanbul, and it doesn't work for us, and it does not appear that it'll get updated any time soon. CodeCov was an alternative, but I'm not "supposed" to use it, and the ADO code coverage support is just that you hand it a report file...so we need a tool to generate one, which we don't and won't have. So I'm closing this.