[Idea] Simplify internal API

Deno provided a simple core functionality based on web standards + some basic features: https://deno.land/api@v1.40.5

We could do the same. For example, we could expose `Bitmap32` and functions for reading standard formats, and that's it.

KorGE:
* basic module <- here basic functionality
* korge-renderer <- here only crossplatform rendering, input without views and basic functionality like Bitmap32
* korge <- the standard korge depending on korge-renderer with views
* korim, korau, etc. <- depending on basic module + providing the current functionality keeping the API as it is right now

Read the issue's Deno API reference and map the current KorGE modules: basic module, korge-renderer, korge, korim, and korau. Determine which basic functionality and dependencies belong in each layer, while preserving the existing APIs provided by korim and korau. Done means the proposed internal API boundaries and module dependency structure are defined and agreed.

Written by the indexing model from the issue text.