Add JS coverage command

[beeman]

Add a Vitest coverage script in the JS workspace and enable HTML and text coverage reports for package source files.

Ignore the generated coverage directory and add the V8 coverage provider so the report can be opened locally from js/coverage/index.html.

This is useful to get visual overview of what code needs more testing.

To use this, run the following command from the root of the repo:

pnpm run -C js test:coverage

Then you can view it:

# open it over the filesystem
open js/coverage/index.html
# host it over http
pnpx serve js/coverage/