feat: Per-tabpage outlines

Closes #37

Almost completely refactored the UI parts outline.nvim to use a Sidebar
object that implements an outline window. In init.lua, we can then store
a table of the outline for each tabpage ID.

When tabs are closed the outline is closed and sidebar reset
responsibly.

This simplifies `init.lua` quite a lot, making it the highest level
control center for the outline elements.

All lua APIs and commands should work as before.

lua/outline/config.lua

@@ -26,7 +26,7 @@ M.defaults = {
     auto_set_cursor = true,
     auto_update_events = {
       follow = { 'CursorMoved' },
-      items = { 'InsertLeave', 'WinEnter', 'BufEnter', 'BufWinEnter', 'TabEnter', 'BufWritePost' },
+      items = { 'InsertLeave', 'WinEnter', 'BufEnter', 'BufWinEnter', 'BufWritePost' },
     },
   },
   outline_window = {