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/code_action.lua

@@ -1,11 +1,11 @@
-local main = require('outline')
+local outline = require('outline')
 
 local M = {}
 
 function M.show_code_actions()
   -- keep the cursor info in outline and jump back (or not jump back?)
   local winnr, pos = vim.api.nvim_get_current_win(), vim.api.nvim_win_get_cursor(0)
-  main._goto_location(true)
+  outline.current:_goto_location(true)
   vim.lsp.buf.code_action()
   vim.fn.win_gotoid(winnr)
   vim.api.nvim_win_set_cursor(winnr, pos)