Proposal: __deps: ["$Foo.bar"] for individual properties on objects

Example of the hypothetical usage:
```js
foo__deps: ['$FS.cwd'],
foo: function(file) {
work(file, FS.cwd());
},
```
Rather than `$FS` in the above, we depend on `$FS.cwd` specifically.

The goal here is avoid including an entire huge object all the time. We have such large objects in `FS` and `Browser`, for example, and using even a single property forces the entire thing to be added, which this proposal could avoid.

Specifically I found the need for this in WasmFS. WasmFS has much less code in JS than the original FS, but there are many JS APIs that call `FS.*` APIs, and right now that is very cumbersome. One option has been to rewrite JS code in wasm to avoid JS deps, and I tried that for `emscripten_async_wget` [in a branch](https://github.com/emscripten-core/emscripten/compare/wasmfs.async_wget?expand=1), but it turns out to be much less pleasant to write that code in C or C++ than JS (simply because JS has such nice ergonomics for async).

This should not cause any breaking changes. It would only influence new code that uses such deps. Implementation-wise, the deps system would special case a dep of a string `X.Y`. That already can't be the name of a JS object, so such deps have never existed, unless I'm missing something.

This would take a little work in jsifier, since we'd only be able to emit an object X after knowing all properties that it uses. But I hope that won't be too bad.

I would use this for WasmFS initially, but we could also apply it to `Browser` and other things and potentially get some nice code size wins.

Thoughts? Alternatives?