Create template for umbrella issues (#134235)

In order to better track features, I propose that we have a unified
template for creating Umbrella issues. This is not required to create
umbrella issue to add to the feature tracker, however for those who
would like a more fleshed out format, this will serve the purpose.

Fixes: https://github.com/flutter/flutter/issues/134234

.github/ISSUE_TEMPLATE/10_feature.yml

+name: Create an umbrella issue
+description: As a contributor, I would like to create an umbrella issue for a
+feature to be added to the feature tracker.
+title: '☂️ <Feature Title>'
+projects: ["flutter/82"]
+body:
+  - type: input
+    id: design_doc
+    attributes:
+      label: Design Doc
+      description: Add a design doc if there is one for this feature.
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: Please describe the feature you are adding.
+    validations:
+      required: true
+  - type: textarea
+    id: tracking_issues
+    attributes:
+      label: Tracking Issues
+      description: List of issues associated with this feature.
+    validations:
+      required: true