idl-api: add idl_get_basetypes()

idl/idl_api.ysl2

@@ -31,6 +31,10 @@ def "func:idl_get_package_license" {
     result "/pEpACIDLang/package/license/text()"
 }
 
+def "func:idl_get_basetypes" {
+    result "///basetypes/*"
+};
+
 
 def "func:idl_typekind_is_defined" {
     param "typekind";