Add theme.json support

[aurooba]

Closes #8

This is a pretty awesome PR, even if I do say so myself. There are a few different things happening here:

• I've added link color support to both blocks
• the list-item width setting no longer sets explicit inline size, and rather modifies a CSS Custom Property inline instead
• there is theme.json support for not only the list-item width setting, but for all the settings the block supports, so any theme can set their own defaults for the Super List block.
  • this includes smart support for the orientation setting! if theme.json sets a default for the list orientation, it will be respected. So if a theme primarily wants to use Super List in horizontal mode, they can set that as their default so they don't keep having to switch it in every new block.
• we've also got a handy dandy theme.json example in the block itself to make life easier for theme authors