feat: add add-platform command

[szymonrybczak]

Summary:

In this Pull Request I've added new command add-platform which re-uses code from init command to add a new OOT platform to existing React Native app. It gets React Native version from package.json and it tries to find a matching platform's version from NPM registry. Then, if successful it reuses editTemplate functions to replace placeholders.

demo.mp4

For OOT platforms to add a compatibility with the new command:

1. Add platforms inside template.config.js, which is an array in which OOT platform should provide names of folders containing native code. e.g. platforms: ["macos"]
2. Add postInitScript (also under template.config.js) with e.g. installing Pods and printing run instructions.
3. Add platform's specific native code under packages/react-native/template${platformName}.

Checklist

• Documentation is up to date to reflect these changes.
• Follows commit message convention described in CONTRIBUTING.md

[szymonrybczak]

cc @kelset @tido64 @Saadnajmi would love to hear your thoughts about it 🙏

[kelset]

looping in also @acoates-ms and @chrisglein who might be interested in this

[TMisiukiewicz]

just a few minor comments, overall lgtm 👍

[Saadnajmi]

At its current point, this doesn't work with React Native Windows or macOS, correct? Has this been tested maybe with some local changes to a clone of RNM/RNW to see how well it works? I think it's harder for me to give any feedback (at least macOS specific) without seeing an end to end use case (other than visionOS :P )