Move most functionality of `UiKitView` and its supporting classes into
superclasses named `DarwinPlatformView`, etc., and create trivial or
near-trivial subclasses with the same names as the old classes.

I am currently awaiting approval for a macOS workstation that would
allow me to run the iOS/macOS tests and make sure all existing
functionality is preserved by this refactor. I can ensure that tests
will pass, but doing so may need to wait for a while.

Addresses [Add
AppKitView](https://github.com/flutter/flutter/issues/128519)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
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[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
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[Discord]: https://github.com/flutter/flutter/wiki/Chat

---------
Co-authored-by: Loïc Sharma <737941+loic-sharma@users.noreply.github.com>
Co-authored-by: Michael Goderbauer <goderbauer@google.com>
Co-authored-by: Chris Bracken <chris@bracken.jp>