feat: use queueMicrotask for nextTick timerFunc

[HOUCe]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Hey guys, thanks for reviewing this.

I've added queueMicrotask for nextTick timerFunc.

Reason to do this:

• queueMicrotask() method is the standard way to introduce a microtask safely and without tricks.
• Why it has the priority than Promise?
  By introducing queueMicrotask(), the quirks that arise when sneaking in using promises to create microtasks can be avoided. For instance, when using promises to create microtasks, exceptions thrown by the callback are reported as rejected promises rather than being reported as standard exceptions. Also, creating and destroying promises takes additional overhead both in terms of time and memory that a function which properly enqueues microtasks avoids.

I think there is no need to create a unit test, current tests will cover this small change well. I also mocked UI experiment for test queueMicrotask() on real terminal, no bizarre behaviors found so far (as it is the standard way).

[HOUCe]

Cloud someone give some attention to this little PR?

@yyx990803
@posva
@eddyerburgh
@Justineo

[Justineo]

LGTM except that dist files shouldn't be included.

[HOUCe]

LGTM except that dist files shouldn't be included.

@Justineo

Sure, dist files is clear now.
I will let you guys to decide the next. Anyway is a tiny change (:

Thanks for the time.

[Justineo]

I've already approved this earlier...@HOUCe