Ability to read keys from the whole window on WASM

I'm unsure if this is a missing feature in bevy, winit, or some other dependency. Presumably someone here will at least know which other dependency I should yell at.

## What problem does this solve or what need does it fill?

My "game" takes up only half the screen and i expect users to spend a lot of time clicking between the canvas and the rest of the webpage. I'd like keybinds to work when the game canvas isn't focused. so you can just hold Q instead of having to click in the canvas area *then* hold Q

## What solution would you like?

An enum resource you can insert:
```
app.insert_resource(WebKeyboardMode::Global);
```
if this is a missing feature in one of bevy's dependencies then we'll have to wait for the dependency to add it, then WebKeyboardMode (placeholder name) would control it.