update the gitattributes for generated code (#1796)

Author: devoncarew

.gitattributes

@@ -1 +1,9 @@
 *.dart eol=lf
+
+# Don't collapse in github code reviews by default.
+lib/src/version.dart linguist-generated=false
+lib/src/html/resources.g.dart linguist-generated=false
+
+# Do collapse in github code reviews by default.
+testing/test_package_docs/** linguist-generated=true
+testing/test_package_docs_dev/** linguist-generated=true