Cleanup peer dependencies

[miltoncandelero]

• Loader packages relied on @pixi/app and resource-loader which were not used.
• Added @pixi/extensions to be compatible with 6.5.1
  • Added a peerDependenciesMeta to make @pixi/extensions optional since is not needed before 6.5.1

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After an npm install gave me errors on my project that doesn't rely on app, I investigated and found that neither @pixi/app nor resource-loader were being used in the code.
I also added the optional new peerDep of @pixi/extensions to be compatible with @pixi/core version 6.5.1

[miltoncandelero]

After talking with @andrewstart we came to the conclusion that I shouldn't have the burden to track my dependencies' dependencies.

I will move @pixi/extensions to devDependencies

I will also check which peerDependencies are actually being used 🤔 just to be extra sure and tackle that chore

[miltoncandelero]

Interesting note about spine-base dependencies:
"@pixi/runner": "^6.1.0", ,"@pixi/settings": "^6.1.0", and "@pixi/extensions": "^6.1.0" are only needed to generate the typescript definition file so they were moved to devDependencies

[miltoncandelero]

Goooot'em!

This latest commit ensures that we never forget any peerDep from our dependencies.

There is probably a better way of doing this, but this works 🤷

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Now I think we are good to merge 👍
UMD Builds seem to be hash perfect compared to the previous one so I think everything is fine 😅