[Process] Add template for breaking change requests

Breaking change requests are currently received in an unstructured way.  To create a more reliable and predictable process, we will use github forms.

This CL creates the form, follow-on CL will update the breaking change documentation.

Issue: https://github.com/dart-lang/sdk/issues/51801

Change-Id: I5ecaa042e3fafdd9d735dd43bccf954aa73e2a2d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290605
Commit-Queue: Kevin Chisholm <kevinjchisholm@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

.github/ISSUE_TEMPLATE/3_breaking_change.yml

@@ -0,0 +1,28 @@
+name: Propose a breaking change.
+description: You would like to propose a breaking change.
+title: '[breaking change] <title>'
+labels: ['breaking-change-request']
+assignees:
+  - itsjustkevin
+  - kevmoo
+body:
+  - type: textarea
+    id: change_intent
+    attributes:
+      label: Change Intent
+      description: What is the intent of this change?
+  - type: textarea
+    id: justification
+    attributes:
+      label: Justification
+      description: What is the justification for this change?
+  - type: textarea
+    id: impact
+    attributes:
+      label: Impact
+      description: What is the impact of this change?
+     - type: textarea
+    id: mitigation
+    attributes:
+      label: Mitigation
+      description: How can we mitigate the impact?