Add a small emoji picker/selector example

[DJMcNab]

This depends on:

• add emoji family for emoji clusters parley#56; AND
• Render COLR Glyphs (Emoji) vello#615; AND
• Support bitmap fonts vello#641

I have added those as our dependencies; it's unlikely that a release containing those will happen very soon.

Some notes:

1. The accessibility of this example probably isn't great. Not sure what to do about this.
2. Our layout primitives aren't great; I've hacked around a grid layout, but we really need to make a Grid widget/View
3. The pagination is slightly hacked together - it should really try and give you page numbers. I'm not planning to address this, unless someone provides the algorithm
4. Emoji only render on systems with a system COLR Emoji font. I know my Android device does, but I don't know if any other systems do? Maybe Windows and some Linux systems? This blocks this PR

This is created for linebender/linebender.github.io#56

[jaredoconnell]

I tested this, and the fonts are not visible on Mac OS. Maybe the font isn't COLR format.

[DJMcNab]

Indeed. This is called out in linebender/linebender.github.io#56, for example.

[DJMcNab]

Moving this into draft due to the lack of bitmap emoji support.