ref: `local m` → `local groups`

[Iron-E]

This PR has a few advantages:

1. Uses the naming scheme seen in other modules
   • i.e. local m → local groups

2. Creates a clear naming difference between group_clumps and group:	old name	new name
   barbar.render.group	barbar.groups.item
   barbar.render.group[]	barbar.groups
   barbar.render.group_clump	barbar.render.group

3. Alleviates cyclical resolution of @params in the groups module
   • e.g. almost all of the params were barbar.render.group[], but barbar.render actually depends on barbar.groups, so it would be cyclical in a traditional type checking system
4. Performance increase by using for i = x, y, -1 instead of utils.list_reverse

[Iron-E]

Ok, I've revised the PR based on your feedback. Summary:

• Clarified group / group_clump:	old name	new name
  barbar.render.group	barbar.ui.node
  barbar.render.group_clump	barbar.ui.container

• We have a few modules that work closely together in order to draw the barbar. I've moved them to a new sub-module called barbar.ui (e.g. barbar.render → barbar.ui.render).
  • This is to create a clearer picture of which modules control the UI for when designing #426