Add build and run commands

[TimJentzsch]

Objective

Easily build and run your Bevy app.
Especially for the web, it can be complicated to get everything set up.

Solution

This code is ported from https://github.com/TimJentzsch/cargo-bavy, but cleaned up, with more docs, and sometimes adjusted to improve the functionality.

For the most part, bevy build and bevy run are wrappers around the cargo counterparts.

However, we add an additional --web flag, which simplifies the process of building and running web apps.

Currently it:

• builds WASM targets
• passes flags along to cargo
• runs wasm-bindgen
• starts a local server

Assets are served automatically from the assets folder.
Missing targets and CLI tools are installed automatically (if confirmed by the user).

TODO

• Make code more robust to different repository setups (e.g. different cargo project structure, different asset folder location, etc).
• Keep the bindgen in the target folder, so it's cleaned up properly and doesn't require an additional gitignore.
• Serve a default index.html file if none is defined, making the web folder fully optional.
• Provide an option to automatically open the browser when running the web build.
• Figure out what to do with the web stuff in release mode.
• Use a sub command instead of flag for the web option. This makes it more flexible to provide web-specific options and help text.
• Support the feature flag and forward it to cargo (can be stacked).
• Split the PR into smaller pieces to make it easier to review.

[TimJentzsch]

This can potentially be replaced by parsing the output of the cargo metadata command

[janhohenheim]

Can this run another binary than cargo? On Windows, you need to run cargo-clif in order to use cranelift.

[TimJentzsch]

Can this run another binary than cargo? On Windows, you need to run cargo-clif in order to use cranelift.

That's not supported at the moment, I wasn't aware of that.