fix(form-plugin): introduce conditional debounce

[FortinFred]

make the form-plugin dispatch actions immediately when the form's updateOn is 'blur' or 'submit' or when the ngxsFormDebounce is less than zero

PR Checklist

Please check if your PR fulfills the following requirements:

• 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?

[X] Bugfix
[ ] 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?

#365

Issue Number: 365

What is the new behavior?

When a ngxsFormDebounce is less than zero or when a FormGroup's updateOn property is configured with 'blur' or 'submit', the debounceTime operator will be remove from the valueChanges and statusChanges pipes.

It seems that even with a timer of zero, the state is updated asynchronously after the form has been submited. Removing the operator in the describe situation resolve the issue.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

[splincode]

Please add tests to your new functionality.

[markwhitfeld]

Great work! Thanks for the contribution.
@FortinFred Do you think there could be any scenario where an application would rely on the broken behavior?
cc @splincode @arturovt

[FortinFred]

@markwhitfeld
No, I can't really see any scenario where this would be disarable.

[FortinFred]

@markwhitfeld
would also add that I understand the reason for the debounce mechanism to exist on the ngxsForm but such feature IMO would better be served by Angular itself.
angular/angular#19705

If the linked FR ever gets implemented, the ngxsFormDebounce would be deprecated and actions would be dispatched synchronously,

[markwhitfeld]

@FortinFred Do you think we need any updates to the docs around this?

[arturovt]

LGTM

[FortinFred]

@markwhitfeld
I think it might make sense mentionning that the ngxsDebouce is ignored when the form's onUpdate === 'blur | submit'

[splincode]

Good job!