Move @expo/config-plugins to peer/dev dependencies

[friederbluemle]

Problem

@expo/config-plugins is currently a direct dependency, which means it gets installed for all users - including those with bare React Native apps who don't use Expo.

Solution

Follow the industry standard pattern (e.g. used by Sentry and React Native Firebase):

1. Remove @expo/config-plugins from dependencies
2. Add expo as an optional peer dependency

Why this works

• @expo/config-plugins is a transitive dependency of expo
• Expo users already have expo installed, so the plugin code can import from @expo/config-plugins
• Non-Expo users won't have expo installed, but they also won't use the config plugin code at all
• The plugin is only invoked during Expo's prebuild phase, which only runs in Expo projects
• For local library development, add @expo/config-plugins to devDependencies, so that imports resolve

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[friederbluemle]

I have read the CLA Document and I hereby sign the CLA

[friederbluemle]

recheck

[FranAguilera]

@friederbluemle apologies about the delay and thanks for you contribution!

This is a nice improvement though we are investigating why our integration tests keeps failing just on ci (same tests passes locally). We'll continue looking and provide an update very soon.

[friederbluemle]

Sounds good, thank you!

[jacksonhardaker]

Sorry for the delay @friederbluemle. TLDR, the e2e test job is failing due to the fact that CI appears to be running within the context of your forked repo, which is missing necessary secrets. We'll look into a more permanent solution, but for now I've duplicated your PR in #193 so that we can get it merged.