Type declarations for walt-loader.

# Feature Request
Today I found that `walt-loader` doesn't work seamlessly in a webpack environment using typescript because its missing type declarations.

This is the type file I've used:
```ts
type WaltInstantiate = (deps?: any) => Promise

declare module "*.walt" {
const value: WaltInstantiate
export default value;
}
```

where `WebAssembly.ResultObject` is defined in `@types/emscripten`.

Start by locating the walt-loader package and its webpack integration or entry point, then compare its TypeScript behavior with the declaration shown in the issue. Done means a TypeScript webpack project can import a .walt module without missing-declaration errors and receives the described default export type.

Written by the indexing model from the issue text.