Improve the @pixi/runner developer experience

[koteelok]

Description of change

The current implementation of the `runner' package is not always usable with classes and can cause bugs due to overly complicated generics.

I removed default values of Runner generics, fixed improper use of unknown type and made types smaller to make errors more readable.

Also i found a bug(?) in BufferSystem caused by lack of strict checks that the object contains a callback function. I didn't add these checks myself so as not to change the APIs of this class too much, but this change may help us avoid similar problems in the future.
Let me know if it's OK to add them, I'll include them in this PR.

Instances problem

before

now

Explicit generics

before

now

Pre-Merge Checklist

• Tests and/or benchmarks are included
• Documentation is changed or added
• Lint process passed (npm run lint)
• Tests passed (npm run test)

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[reececomo]

If you use any kind of code/property mangling for production bundles you might want to be passing this.myFunction.name instead of raw strings