Add issue template pointing to stack overflow

Add a `config.yml` which includes a stack overflow link as "contact" to
encourage general help questions to get help from the community first.

Enable blank issues because they are currently allowed.

Change-Id: Ifc43e36cd022a0eafa6208f26eb618e1a27c791c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/397100
Auto-Submit: Nate Bosch <nbosch@google.com>
Reviewed-by: Devon Carew <devoncarew@google.com>
Commit-Queue: Nate Bosch <nbosch@google.com>

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,7 @@
+blank_issues_enabled: true
+contact_links:
+  - name: I want help writing Dart code
+    url: https://stackoverflow.com/tags/dart
+    about: >-
+      Visit Stack Overflow for how-to questions related to using the Dart
+      language, core libraries, or developer tools.