Implement online/offline detection in browsers

[matheus23]

We currently disable network change detection in wasm32-unknown-unknown.

Instead, we should subscribe to the browser window's online and offline events.

For that, we simply implement online/offline event handling in netwatch.

Then we can remove network_monitor-related cfgs in magicsock::Actor::run.

[nosideeffects]

Preferably target globalThis.navigator so this can also support Web Workers.
WorkerNavigator

Snippet from MDN on global object:

console.log(globalThis === globalThis.globalThis); // true (everywhere)
console.log(window === window.window); // true (in a browser)
console.log(self === self.self); // true (in a browser or a Web Worker)
console.log(frames === frames.frames); // true (in a browser)
console.log(global === global.global); // true (in Node.js)

[matheus23]

Yeah - thanks @nosideeffects. An implementation of this now already exists in #3206 and it uses the js_sys::global() function, which includes various fallbacks, such as globalThis, and (new Function("return this"))(), too, so we'll have that covered ✌️