Add configuration to automatically disable/enable features when running in the web

[TimJentzsch]

We should provide an option to automatically disable default features and enable features when running the app on the web.

For example, via a package.metadata.bevy_cli.web.default-features = false configuration in Cargo.toml.

Use case: Some features, like file watching, are only available on native platforms.
It's convenient to have them enabled by default, so that cargo run gives you hot asset reloading.

But then we have to always add --no-default-features when running bevy run web, which is a regression in TheBevyFlock/bevy_new_2d#312.
It would be nice if we could permanently configure it instead so the user doesn't have to type it out every time.

CLI options should take precedence over the Cargo.toml configuration.