Simpler internals for dual-implementations

[iwoplaza]

Whenever we create a function that works in both JS and generates shader code, we rely on it to choose behavior based on the context (if codegen mode, then we assume the inputs are snippets and we return a snippet, otherwise we execute the code, for example).

If we instead make direct calls always execute in JS, and store the codegen-impl as a prop, we might be able to make it more performant and intuitive.

[reczkok]

We could move the simulate mode out of the TypeGPU package into a separate one - this would simplify the runtime logic and makes the most sense imo