chore(repo): Add internal blank issue template (#12986)

With the new issues UI (beta), our previous way of skipping issue
templates for internally created issues doesn't work anymore. So we
add a new template that essentially mimics a blank issue.

.github/ISSUE_TEMPLATE/internal.yml

@@ -0,0 +1,9 @@
+name: 💡 [Internal] Blank Issue
+description: Only for Sentry Employees! Create an issue without a template.
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+    validations:
+      required: true