[share_plus] Provide a way to check for sharing availability

[doppio]

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. 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.

[Coronon]

Hi there, thanks for your suggestion.

When it comes to sharing, there are currently two basic types: pure text or files.
While most platform support some kind of sharing, we would need to differentiate here. Sharing is basically intended for mobile platforms, and all others are pretty much a "best effort". We could probably implement this quite easily with something like this. The question really boils down to giving any guarantees on platforms like Web and Desktop (apart from maybe macOS as they actually really have a sharing API).

My personal opinion is that we should stop supporting all platforms where sharing is not "natural" → provide good feature support and availability to iOS, Android and macOS (but that in itself is controversial in this repos' community).