Extract Syntax highlighting from TextEdit and add EditorSyntaxHighlighter

- Extacted all syntax highlighting code from text edit
- Removed enable syntax highlighting from text edit
- Added line_edited_from signal to text_edit
- Renamed get/set_syntax_highlighting to get/set_syntax_highlighter
- Added EditorSyntaxHighligher

editor/plugins/visual_shader_editor_plugin.h

@@ -72,6 +72,7 @@ class VisualShaderEditor : public VBoxContainer {
 	bool shader_error;
 	VBoxContainer *preview_vbox;
 	TextEdit *preview_text;
+	Ref<CodeHighlighter> syntax_highlighter;
 	Label *error_text;
 
 	UndoRedo *undo_redo;