Add optional test coverage reports

[dcodeIO]

Based on #2294, but keeps coverage manual / optional for now.

cc @JesseCodeBones: Wdyt?

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[JesseCodeBones]

@dcodeIO
I think we need add two things:
1, enable all features during coverage: Link
2, add c8 to the npm dependencies: Link, maybe the package_lock.json also need to be included.
you can chose merge which PR to be merged, Thank you for your advice:)
PR

[dcodeIO]

The idea is to provide a start for those interested in working on coverage. In the typical case of normal development, the additional coverage dependencies are not necessary, hence keeps c8 on-demand. I've documented the remaining bits for now, including to manually enable all (relevant) features. Should get us going, and we can still extend this later :)