Update API - Githubissues

This PR is part of the effort to improve Node's support for code coverage (see discussion on c8).

This PR exposes two new functions to support foreground childs:

proxy(parent, child) to start forwarding between two existing processes.
spawn(file, args, options) as a replacement for the original functions allowing you to pass spawn options.

The original API is still there. The only breaking change is dropping support for unmaintained Node versions (require Node 6+).

The new spawn function would allow to pass the env NODE_V8_COVERAGE variable directly in c8. There are a few other changes:

It always defaults to use the builtin cp.spawn function (even on Windows), you can pass your own function if you want to.
It does not take over control flow: it is your responsibility to exit the parent process once the child is closed.

Other comments:

I converted the lib to Typescript (for documentation). I also converted the tests to make sure the types work fine.
I completed the test suite to reach 100% coverage.
From a semver point of view, this is a major version.
I update the dependencies.

Discussions:

Maybe provide a way to automatically kill the parent process like in the original function?
Migrating from the original function to fg.spawn should be relatively easy. For the moment I marked it as deprecated, I don't think it should be removed immediately (especially since it's known to work fine with nyc). Not sure what you think about it.
Closes tapjs/foreground-child#13
Closes tapjs/foreground-child#14
Closes tapjs/foreground-child#15
Closes tapjs/foreground-child#16
Closes tapjs/foreground-child#17
Closes tapjs/foreground-child#18
Closes tapjs/foreground-child#19

tapjs / foreground-child