Enhancement: Generate test coverage report after npm test

[JesseCodeBones]

Currently, the project do not have test coverage feature, and we donot know if the test is enough or not.
So I suggest if we can have test coverage feature, it is better for us to control test or CI flow in the future.
I will introduce C8 here:
Here is the website and documentation of C8:
https://www.npmjs.com/package/c8
C8 can generate html report, text report, cobertura report and etc.
Here is report screenshot:


it also report directly during test run:

The report will generated under ./coverage folder, of cause it should be git ignored.

[JesseCodeBones]

I created one PR, please kindly review and decide if we need this feature, thank you:
#2294

[JesseCodeBones]

I skiped the lib folder and created a new PR, please kindly check, thanks.

[dcodeIO]

Optional code coverage has been added in #2517 in spirit of #2294. There's some documentation. Do you want to keep this issue open to track further progress, or can we close?

[JesseCodeBones]

@dcodeIO
I will close it, for the test progress, I think it is suitable to divide them with different functionalities.