ref: `bufferline` → `barbar`

[Iron-E]

This moves all logic out of the bufferline lua module and into the barbar module.

• This does not deprecate bufferline. We have deprecated quite a few things recently, so we should wait before deprecating more.
  • This will make it easier to eventually deprecate the bufferline module.
• This does update all documentation to refer to the new module location.