Support feature detecting if an API is available

[fitzgen]

To get cross-browser functionality working, JS code often does things like

if (typeof fetch === "function") {
  // assume `fetch` API exists
} else {
  // fall back to XMLHTTPRequest
}

We should support this kind of feature detection somehow.

---

Strawman: add a #[wasm_bindgen(feature_detect = function_name)]-style attribute:

• function_name would be the name of a function with signature fn() -> bool that returns true if the thing was found in JS and false otherwise

• If the imported thing was not found in JS, we bind to a stub that throws + logs a JS error.

• enables usage like this:

#[wasm_bindgen]
extern {
    #[wasm_bindgen(feature_detect = is_fetch_available)]
    fn fetch(...) -> ...;
}

if is_fetch_available() {
    fetch(...);
} else {
    // do some fall back...
}

Perhaps this requires an RFC.

[alexcrichton]

This got me thinking! I managed to get rust-random/rand#541 working with what we've got today on crates.io!

I'm not entirely sure how much we want to bake in, from what I've seen everyone does feature detection slightly differently so we may want to just be sure to enable various patterns rather than trying to bless one?