feat: Proper floating window for keymap help and OutlineStatus

No more obnoxious '}' on the cmdline when pressing `?`!

scope:
- More type hints
- Added class Float for creating floating windows with size that fit the
  content and position centered on the screen
- show_help action for outline window (key `?`) now uses a floating
  window
- :OutlineStatus now provides better information, and shows content in a
  floating window.

future:
- Floating window option configuration

lua/outline/providers/init.lua

@@ -3,8 +3,6 @@ local cfg = require('outline.config')
 local M = {}
 local import_prefix = 'outline/providers/'
 
-_G._outline_current_provider = nil
-
 function M.find_provider()
   if not M.providers then
     M.providers = vim.tbl_map(function(p)