feat(editor): introduce split pane manager plugin and refactor source view system

* address TODO item for split_pane_manager plugin and bound keys
* add split_pane_manager plugin with create/close split view commands
* move sibling focus/move commands from core into plugin system
* remove legacy toggle_source_view plugin
* redesign EditorsContainer to use simpler Box-based layout
* refactor source view API to return (scrolled_window, source_view)
* add source view lifecycle events (create/remove/removed)
* rename GetNewCommandSystemEvent → CreateCommandSystemEvent
* update CommandsController to support command system creation and cleanup
* ensure command systems are removed with their source views
* improve focus border handling across sibling chains
* update telescope integration for new source view structure
* clean up container imports and initialization logic
* remove old keybindings and align with new split pane workflow

TODO.md

@@ -2,7 +2,6 @@ ___
 ### Add
 1. Add Godot LSP Client
 1. Add Terminal plugin
-1. Add Plugin to <Shift\><Ctrl\>| and <Ctrl\>| to split views up, down, left, right
 1. Add <Ctrl\>i to **lsp_manager** to list who implements xyz
 
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