js: Promisify async operations

Promises make asynchronous operations easier to manage, in particular when used through the async/await syntax that allows for asynchronous code to closely resemble synchronous one.

gjs has included a Gio._promisify() helper for a while now, which monkey-patches methods that follow GIO's async pattern to return a Promise when called without a callback argument.

Use that to get rid of all those GAsyncReadyCallbacks!