Make it easier to pass inputs through the builder API

[Michael-F-Bryan]

The current withCapability() method takes a function that constructs a Capability object who's properties are incrementally updated... This is a massive pain for frameworks like React that hate mutation.

Something like this might be easier to work with:

type InputDescription = {
  type: "raw" | "image" | ... | "sound",
  args: Record<string, number>,
};

type GetInputs = (InputDescription[]) => Tensor[];

class Builder {
  ...

  // Set the function Rune will call when it wants inputs.
  withInputs(getInputs: GetInputs): this { ... }
}