templates: Allow using promises in GJS

Promises and by extension async/await is not usable without this.

See gjs#468 (closed)

I know this is not great to look at but it's the only option to make async JavaScript work as expected. We are discussing options to have a helper for this in GJS but we'll have to wait a couple of cycles before we can use it in GNOME Builder template anyway.

Here is a relevant Matrix discussion https://matrix.to/#/!pOGgkAueAsusiyFCTb:matrix.org/$1646179524313902WKRJi:matrix.org

cc @ebassi @ptomato