Add issue template and other fixes

Author: mosuem

.github/ISSUE_TEMPLATE/string_scanner.md

@@ -0,0 +1,5 @@
+---
+name: "package:string_scanner"
+about: "Create a bug or file a feature request against package:string_scanner."
+labels: "package:string_scanner"
+---

pkgs/string_scanner/pubspec.yaml

@@ -1,7 +1,7 @@
 name: string_scanner
 version: 1.4.0
 description: A class for parsing strings using a sequence of patterns.
-repository: https://github.com/dart-lang/string_scanner
+repository: https://github.com/dart-lang/tools/tree/main/pkgs/string_scanner
 
 environment:
   sdk: ^3.1.0