[share_plus] Provide a way to check for sharing availability

## Use case

I have a cross-platform app with a share button, but sharing doesn't work on all platforms. For example, attempting to share in Chrome on macOS just opens a blank tab, as [some browsers do not support sharing](https://caniuse.com/mdn-api_navigator_share). I would like to be able to hide the "Share" button if it's not an option on the current platform.

## Proposal

Expose something like an `isSharingAvailable` method which would return true on most platforms, but can return false in scenarios such as the one mentioned above.