Set up minimal "hello world"-like bevy project

[benfrankel]

Include:

• src/main.rs with App::new().add_plugins(DefaultPlugins).run()
• Cargo.toml that depends on bevy; disables the 2 common false-positive lints; and enables optimizations
• .gitignore

[benfrankel]

This can come from https://github.com/richchurcher/bevy_scaffold/.

[benfrankel]

This can be considered complete now :)