add overload to filter with typeguard predicate

[Milly]

We can convert the type of item by specifying a type-guard function to filter.
Note that this is not available for async filter functions, because Typescript does not have async type-guard syntax.

import { asynciter } from "./asynciter.ts";

const isString = (item: unknown): item is string => typeof item === "string";

const iter = asynciter(["foo", 42]); // => IAsyncIter<string | number>

const stringIter = iter.filter(isString); // => IAsyncIter<string>