Once the shell is sufficiently mature, we need to support creation of a Linux runner from a template using flutter create.

The lack of a de-facto standard toolchain means we'll need to figure out what the default runner will use, both in terms of compiler (gcc? clang? try to support both?) and build system (make? cmake? gn+ninja?), as well as whether we'll require them to be installed by the developer, or use internally-managed copies.

Relatedly, we'll need to figure out how to handle runner dependencies. E.g., if the shell ends up being GTK-based, then the runner will have GTK dependencies, which will need to either be managed internally, or covered by flutter doctor