breaking: Bundle iOS and Android as core platforms

[huntie]

Summary

Bundle cli-platform-android and cli-platform-ios in the main @react-native-community/cli package.

Relates to this proposed change: react-native-community/template#235

Motivation

Simplify Community CLI setup for users (see above). iOS and Android are first-class RN platforms.

• While the CLI was historically separated out into granular plugins, I don't believe we need to be so aggressive in splitting the core Android and iOS platforms into separate installs. This optimisation provides little value when RN itself bundles both — match this baseline.
  • Other plugins, including OOT platforms, remain selectively applied via plugin discovery. Again, the redundant weight of ~20 scripts (each way) being omitted for a single platform repo is a marginal win. The convention (in the case of libraries) is that OOT platform support is additional.
  • We can always un-bundle later (in a hypothetical project where RN also separates core from platforms).
• Prior art: Expo CLI doesn't bother separating platforms — the dev install impact on projects is negligible. No related artifacts leak into the project.

Notes

• loadConfig and loadConfigAsync gain an optional platforms argument that seeds the config. Project and dependency configs still layer on top and can override it, and the --platform filter is honoured.

Test Plan

TODO

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).