[WIP] Migrate from istanbul to nyc coverage tool

[brodycj]

Platforms affected

All

Motivation and Context

It is desired to use a supported coverage tool that will be updated and should be less likely affected by security issues in the future.

Description

See title: migrate from istanbul to nyc coverage tool, in a similar fashion to the updates in apache/cordova-lib#637, apache/cordova-lib#783, and apache/cordova-lib#784

Testing

• npm run cover succeeds on supported Node.js versions 6, 8, and 10

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[dpogue]

Actually, it looks like codecov isn't able to find the coverage file anymore...

[erisu]

You might want to take a look how I setup nyc on other repos for example Cordova Electron.

https://github.com/apache/cordova-electron

The package.json contains the nyc reporting configurations and the script that passes in the test configuration file.

This should still create the coverage directory and still be in the .gitignored.

[brodycj]

Thanks guys, marked as WIP for now.