## Description

This PR adds the share button to text selection toolbar buttons on Android ~~and iOS~~ for `SelectableRegion` (and therefore `SelectionArea`).

https://github.com/flutter/flutter/pull/139479 adds this button for `EditableText` (which is used by `TextField` and `SelectableText` but not by `SelectionArea`).

**Edit**: supporting this on iOS will need more work (see https://github.com/flutter/flutter/pull/141447#issuecomment-1889942622 and https://github.com/flutter/flutter/issues/141775).

## Related Issue

Follow up for https://github.com/flutter/flutter/issues/138728

## Tests

Adds 1 test.