feat(effects): createEffect returns specific type for dispatch false

[alex-okrushko]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

based of #2194

createEffect with {dispatch: false} now returns whatever Observable type is actually returned (instead of unknown).

[ ] 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?

createEffect with {dispatch: false} returns Observable<unknown>.

What is the new behavior?

createEffect with {dispatch: false} returns Observable<SpecificType>.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Somewhat. If anyone was relying that Observable<unknown> is returned. However, Observable<SpecificType> can be always widened to Observable<unknown> so it shouldn't really break anything.

Other information

[ngrxbot]

Preview docs changes for dcaf89d at https://previews.ngrx.io/pr2195-dcaf89d/

[alex-okrushko]

Rerunning the tests should fix the issue. (I do not have permissions at circle ci)

[brandonroberts]

@alex-okrushko this one needs another pass on the merge conflicts.