Emoji Shortcode Support

**Is your feature request related to a problem? Please describe.**
Android and the desktop versions of Element support emoji shortcode. However, iOS is without this feature and causes one to waste a significant amount of time inserting an emoji.

**Describe the solution you'd like**
When I type `:ap`, I'd like to see all available emojis that match that shortcode.
When I type `:apple:` and continue typing, I would like my shortcode to convert to 🍎 .

**Describe alternatives you've considered**
I have to manually search the emojis or copy & paste from another source to get the emoji I want.

**Additional context**
N/A