feat: accept function values as valid instances

[esroyo]

There is a lot of JavaScript/browser functionality that comes just shaped as a function and not an object/instance: fetch, setTimeout/clearTimeout, atob/btoa, requestIdleCallback, etc. It seems handy to be able to inject function values on provisions. Does it feel ok? Any thoughts?

interface Fetch {
    (input: RequestInfo, init?: RequestInit): Promise<Response>;
}
const customFetch: Fetch = /* custom implementation */;
class HttpClient { 
    constructor(readonly fetchImpl: Fetch) {}
}

const injector = new Injector([provide(reify<Fetch>(), () => customFetch), provide(HttpClient)]);