[enhancement] consume upstream webview

I know that this has been discussed before, but I think it is time to revisit it for a couple reasons.

First of all, there is now activity again at: https://github.com/zserge/webview/tree/webview-x

I think that it would be beneficial for the entire community if the enhancements made here were pushed back upstream.

Second - at tauri we have the need to enable other languages (like python, nim, go and others) to consume the underlying webview codebase. The way that `boscop/web-view` is structured at the moment makes that challenging, because of the way that the upstream source has been embedded. Basically non-rust consumers of tauri will need to pull in a rust crate / git repo to get the headers etc. in order to stay in lockstep with our api - which we for obvious reasons can't allow to use two diverging sources.

One solution might be to use a flag in the consuming Cargo.toml that allows a user of `boscop/web-view` to pull the source from some / any other place...

Thoughts?