register as share target

One feature that prevents me from using Phanpy as my only Mastodon client on an Android device is that it cannot be the target of a share so doesn't let me share an image as easily as it happens with native applications. But it looks like it could do that by adding some additional directives in the PWA manifest file:

https://developer.mozilla.org/en-US/docs/Web/Manifest/share_target