Add a command for running Bevy for web

[janhohenheim]

The web (through WASM) is a common platform for game jams, but it currently requires jumping through some hoops to get it working with Bevy. We should make this process easier, specifically for during the development cycle.

We could have a bevy web or bevy run --web command that does the following:

• Builds project for wasm32-unknown-unknown target and passes flags along to cargo.
• Sets up a basic index.html if you have none
• Runs wasm-bindgen
• Copies the assets to be served
• Starts a local HTTP server
• Opens the browser

IIRC cargo-bavy already does most of these, so we can borrow code from there.

[BD103]

Just as a note, I edited Jan's original message as part of my issue-pass in #34.

[janhohenheim]

The edit swallowed up one major point: passes flags along to cargo. This is the one thing that is annoying me the most about Trunk, since it means we cannot have dedicated Wasm profiles when using it.
Edited it back in :)