fix(esl-utils): debounced promise return (POC)

[fshovchko]

Closes: #1954
Request was to handle the following code

  class Test {
    @decorate(debounce, 100) // TS: type are incompatible
    protected async update(): Promise<void> {
       // ... await
    }
  }

In order to do so, debouncedSubject now needs to return the promise

[codeclimate]

Code Climate has analyzed commit a054834 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 65.3% (0.0% change).

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[ala-n]

Why do we change the actual return type?
It was made by request to make lazy creation of promise as the user does not need it in 95% of cases.