feat(functional): add scopedSlots to context in functional components

[posva]

Related #5381

We discussed this being added to 2.6.
It is nothing new, as scopedSlots already exist on data. The only reasons to add it are convenience and consistency with slots (although slots is a function).
If you have arguments for or against this feature, please share them 🙂

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:

[posva]

I may have forgotten some scenarios as well, to easy to add to be real 😆
I'm adding the typings