feat: new action handler for child state

[maliouris]

PR Checklist

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• The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior

When creating an abstract class that has an action handler and this abstract class is inherited by two states they both listen to this action.

What is the new behavior?

• Add dispatch options in order to scope an action.
• Now we can use the scope property of the dispatch options and pass a StateClass as a parameter so that only this state handles the corresponding action.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[codeclimate]

Code Climate has analyzed commit 6a3a1c9 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 98.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 97.1% (0.0% change).

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