fix(coverage): add coverageRoot option for monorepo setups

[mfazekas]

Summary

Adds coverage.root config option to fix 0% coverage in create-react-native-library and other monorepo setups.

Usage

// example/rn-harness.config.mjs
export default {
  coverage: {
    root: '..', // Point to parent where library src/ lives
  },
  // ...
};

Fixes #58

[vercel]

@mfazekas is attempting to deploy a commit to the Callstack Team on Vercel.

A member of the Team first needs to authorize it.

[mfazekas]

Perhaps this is a documentation-only issue? Instead of adding a new config option, we could just document that users can set NYC_CWD to change the root directory for coverage:

NYC_CWD=.. yarn test:harness:ios --coverage

This is a standard babel-plugin-istanbul/nyc environment variable and doesn't require any code changes.

Thoughts?

[V3RON]

I believe this should go into the config file. Users shouldn't need to know exactly how coverage is implemented under the hood. I also wonder whether we have any other configuration properties related to coverage. If so, we might consider nesting root under coverage instead of using a flat coverageRoot 🤔