allow `subcommands` to be built/executed lazily

[Schniz]

Something like:

1. dynamic command implementations will allow to lazily load the dependency tree instead of forcing cmd-ts apps to be imported sync

   subcommands({
     cmds: { hello: async () => (await import('./my-command')).cmd }
   })
   

2. async functions for the different commands can allow to lazily bootstrap the CLI

   subcommands({
     cmds: () => Promise.resolve({ hello: ... })
   })
   

3. mixing them both can allow extremely dynamic applications 😮