Add Duplicate Lines shortcut to CodeTextEditor

This keyboard shortcut has been made with inspiration from the VS Code keyboard shortcut editor.action.copyLinesDownAction. It duplicates all selected lines and inserts them below no matter where the caret is within the line.

scene/gui/code_edit.h

@@ -486,6 +486,9 @@ public:
 
 	void set_symbol_lookup_word_as_valid(bool p_valid);
 
+	/* Text manipulation */
+	void duplicate_lines();
+
 	CodeEdit();
 	~CodeEdit();
 };