Add test coverage summary to report comment

[tobiasbueschel]

@mattallty what do you think about adding a summary of the overall test coverage report to the reporting comment, for example like this:

---

File	% Stmts	% Branch	% Funcs	% Lines
All files	91.2	94.15	90.42	81.55

Test coverage detailed results

... detailed results https://github.com//issues/27

[Jacse]

We've done something like that in https://github.com/FirmNav/jest-code-coverage-commenter/blob/master/src/code-coverage.ts which may work for inspiration

[MishaKav]

I find the solution in a similar package
https://github.com/marketplace/actions/jest-coverage-comment