refactor: Reorganize modules

Primarily:
- Utils
- Sidebar (removed the need of writer.lua)
- Resolve keymaps shortcut in config eraly
- Put highlight functions into highlight.lua
- Put functions that do stuff on outline window into view.lua

lua/outline/init.lua

@@ -337,7 +337,7 @@ function M.setup(opts)
   }
 
   cfg.setup(opts)
-  highlight.setup_highlights()
+  highlight.setup()
 
   setup_global_autocmd()
   setup_commands()