Document PropType usage with the Composition API

[skirtles-code]

Closes #1621.

Add documentation about working with complex types for props in the Composition API.

Specifically, PropType isn't currently mentioned on the page about using TypeScript with the Composition API. It only features on the page about the Options API.

The original suggestion, on #1621, was to document this near the top of the page, before we start discussing type-based prop declaration. That was my original plan, but I felt there were a few problems with that approach:

• The page also mentions the props option, which also uses PropType for complex types.
• That earlier section seems to be trying to encourage the use of type-based declaration. Runtime declaration is glossed over very quickly. Adding a more detailed explanation would delay mentioning the 'recommended' approach.
• The docs don't currently show how to use complex types with type-based declaration. It may be 'obvious', but without mentioning it explicitly I think someone could get confused and think they need to do something with PropType.

I also didn't want to repeat everything that's on the Options API page.

With those considerations in mind, I've added a separate section to the end of the section about props. It documents using complex types with the three different approaches. The examples are based on those in the Options API TS docs, but I haven't gone into the same level of detail as those examples. I've included a link to the Options API TS docs for anyone who wants more examples, but I don't think there's any essential information missing.

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