Add preload and PI to GDScript reserved keywords

I've noticed that gd_tokenizer also has keywords for `switch' and `do', but they seem not to be supported actually (only used in commented code for the shader language), so I don't add them.

modules/gdscript/gd_script.cpp

@@ -2687,6 +2687,7 @@ void GDScriptLanguage::get_reserved_words(List<String> *p_words) const  {
 		"float",
 		"int",
 		"null",
+		"PI",
 		"self",
 		"true",
 		// functions
@@ -2695,6 +2696,7 @@ void GDScriptLanguage::get_reserved_words(List<String> *p_words) const  {
 		"class",
 		"extends",
 		"func",
+		"preload"
 		"setget",
 		"signal",
 		"tool",