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/shader_editor_plugin.h

@@ -44,6 +44,7 @@
 class ShaderTextEditor : public CodeTextEditor {
 	GDCLASS(ShaderTextEditor, CodeTextEditor);
 
+	Ref<CodeHighlighter> syntax_highlighter;
 	Ref<Shader> shader;
 
 	void _check_shader_mode();