- In the process, refactored focusable_preview to a util function.
- Add text for locations_to_items of the current line.
- Improve location callback to handle multiple return values by using
set_qflist.
- Remove update_tagstack and leave note for future travelers.

Add set_qflist and set_loclist.
- Also add locations_to_items, which calculates byte offsets for
character positions in files and avoids unnecessary operations.

- Implements textDocument/formatting, textDocument/rangeFormatting,
workspace/applyEdit.
TODO:
- still has edge cases around replacement probably. Only tested with
inserts on the same position.

- Rename builtin_callbacks to default_callbacks and slightly change its
semantics:
  - No longer contains the default implementations. Instead, any
  default_callbacks will be used in preference for our .buf methods.
  - Add this to the docs.