fix(typings): wrongly typed AsyncComponentFactory

[backbone87]

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:
If people used the wrongly typed part of the changed interface, typescript compiler may complain now. No effect in transpileOnly mode, it only effects the TS typechecker.

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:

the component property of the return value of the
AsyncComponentFactory should be a promise and not a
function returning a promise

[backbone87]

ci errors seem unrelated

[backbone87]

@LinusBorg sorry for pinging you directly, but since there is no response on this, i just wanted to raise someones attension: can this get merged?