Ability To Build/Bundle Multiple Platforms At Once

## Feature Request

Add the ability to specify all or a list of platforms to create builds/bundles from. This would make releasing multi-platform projects as simple as a single command.

## Implement Suggestion

We could make the `--platform` flag support `--platform all` or `--platform web,desktop,fullstack`.

It may also be nice to have a list of platforms in `Dioxus.toml` that a project supports which `dx` will build/bundle from instead of manually specifying `--platform web,desktop,x,y,z`