[WIP] Code coverage

[gogolon]

This adds a possibility to collect code coverage from patrol tests using patrol test --coverage. Unwanted files can be ignored using glob patterns, i.e: patrol test --coverage --coverage-ignore="**/*.g.dart"

~~The PR is not finished yet - there is one thing that needs to be addressed. Currently, the coverage is only collected from isolates that are running when particular test finishes - if an isolate exits prior to this, it will be omitted. The --pause-on-exit flag is already set, but coverage has to be collected from the paused isolate separately, and it has to be resumed (other isolates might be waiting for its result).~~

The code should be now fully functional, but it needs some cleanup

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[pdenert]

@gogolon good job! Please bump versions of patrol and patrol_cli, remember about packages/patrol_cli/lib/src/base/constants.dart and compatibility_table. Also add Changelog for both packages