Feat/scene tree buffer

[Mathijs-Bakker]

Add a scene-tree viewer as available in Godot Editor.

• Open the current scene-tree with :GodotSceneTree in a pane.
• Can be configurated (enabled/disabled) in the plugins config.
• Makes use of nerdfont icons. When a nerdfont is not available then the plugin falls back to ASCII.
• Keymap: y yanks the node path to the clipboard.