feat: Include a command to generate a single coverage report for multi-package apps

[3ettilina]

Description
After running very_good test ... --coverage -r there's no easy way of creating a single coverage report with the lcov files that were generated.

Would be great to have a new option to generate a single HTML report file whilst the tests are running and be able to open (manually or automatically) the report afterwards.

Steps To Reproduce

1. Run very_good test ... --coverage -r
2. The command generates LCOV files inside each package of the app
3. There's no in-CLI way to merge them together in a single coverage report file

Expected Behavior

1. Run something like very_good test ... --coverage **--report** -r
2. The command generates a single report with all LCOV files generated in each package
3. Can be opened by simply running open coverage/index.html or having another command option --coverage-open, which does both things (generating and opening) 🤔

Additional Context
Would be great to see (if possible) the HTML report file as:

• lib
• packages/api_client
• packages/app_ui
• packages/user_repository
• ...

So we can have the whole coverage of each package in the first page, and then tapping on each of those bullet options we can navigate to the coverage of that package.

[erickzanardo]

Kinda of related to: #418