feat(run): add --watch flag

[ratson]

Add --watch flag for ags run.

[Aetherall]

Hi ! I think you can rely on esbuild watch mode and hook the onEnd handler in a plugin to restart the gjs child :) It might save some trouble afterwards, builders know about imports, loaders etc

[Aylur]

https://github.com/Aylur/ags/issues/724