LibGUI: Add Ctrl-U to insert mode

While under insert mode, Ctrl-U deletes all characters between the first
non-blank character of the line and the cursor.

Implement delete_from_line_start_to_cursor() in TextEditor. Then, call
the method in VimEditingEngine via its pointer to an instance of
TextEditor.

Userland/Libraries/LibGUI/TextEditor.h

@@ -137,6 +137,7 @@ public:
     void delete_current_line();
     void delete_previous_word();
     void delete_previous_char();
+    void delete_from_line_start_to_cursor();
     void select_all();
     virtual void undo();
     virtual void redo();