Revise import url handling

[jberdine]

          It's because for vite we use the `await import ` pattern that returns a module with a default property that contains the URL in string format.

It's was a fix made in a bad place to manage that.
The import for Url pattern shall be correctly manage each time it's used. In skipruntime_init.ts it's badly managed the wasmURL function shall returns a Promise<URL | string> and the the default property as string of await import("./libskip-runtime-ts.wasm?url") that should be returned and not the await directly.

Originally posted by @skiplabsdaniel in #421 (comment)