Add automatic refresh and a public interface on top of #23736
* add on_reload, on_detach handlers in `enable()` buf_attach, and
LspDetach autocommand in case of manual detach
* unify `__buffers` and `hint_cache_by_buf`
* use callback bufnr in `on_lines` callback, bufstate: remove __index override
* move user-facing functions into vim.lsp.buf, unify enable/disable/toggle
Closes#18086