Add a `bevy run` command which conveniently runs your Bevy app.

It mostly wraps `cargo run`, but also provides a `web` sub command,
which makes it a lot easier to target the browser.

This option will compile your app for WASM, create JS bindings, add an
`index.html` file if you don't provide one yourself and serves the build
locally to open it in your browser.
The default `index.html` file is mostly what we had in
`bevy_quickstart`.

This is the last part of #24.
Closes #8.

# Testing

1. Checkout the `bevy-run` branch.
2. Run `cargo install --path .` to install this version of the Bevy CLI.
3. Navigate to your Bevy app.
4. Run `bevy run web`.

Note that your app must be compatible with WASM. If you have features or
profiles enabled by default which are not compatible with WASM, you need
to disable them. E.g. `bevy run --no-default-features web`.
If you have a custom `index.html` configured for `trunk`, it might also
not work out of the box. You can try removing the entire `web` folder to
try the default setup.

A good example project is of course `bevy_new_2d`, which you can test on
this branch: <TheBevyFlock/bevy_new_2d#312>