fix(types): no props typings in js files

[Ilanaya]

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

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

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

• [ ] Yes
• [x] No

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

The PR fulfills these requirements:

• [x] It's submitted to the main branch for v2.x (or to a previous version 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)
• [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [x] 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:

This PR intended to fix an annoying problem that caused no type support for props in js files. As it said it ts documentation unspecified types defaults to any which caused the whole props object to become any.

Explicitly specifying the type unknown in generic should fully fix this problem.

References: Similar PR addressing the same issue but with a bit harder solution

Known Vue language tools issues which are closed as upstream because of this bug. https://github.com/vuejs/language-tools/issues/2347 https://github.com/vuejs/language-tools/issues/1537