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/view.lua

@@ -2,6 +2,10 @@ local cfg = require('outline.config')
 
 local View = {}
 
+---@class View
+---@field bufnr integer
+---@field winnr integer
+
 function View:new()
   return setmetatable({ bufnr = nil, winnr = nil }, { __index = View })
 end