📖 Convert feature request issue template to a form (ampproject#34398)

.github/ISSUE_TEMPLATE/feature-request.yml

@@ -0,0 +1,36 @@
+---
+name: Feature Request
+description: Used to report a feature request for AMP.
+labels: "Type: Feature Request"
+body:
+- type: markdown
+  id: header
+  attributes:
+    value: |
+      Thanks for filling out this feature request.
+      - Feature requests related to the [AMP](https://amp.dev) format and cache can be reported using the form below.
+      - Feature requests related to the [AMP Wordpress Plugin](https://wordpress.org/plugins/accelerated-mobile-pages/) can be reported at the [`amp-wp`](https://github.com/ampproject/amp-wp/issues) repository.
+      - Questions about AMP uage can be asked at the [`#using-amp`](https://amphtml.slack.com/archives/C9HPA6HGB) Slack channel or the [`amp-html`](http://stackoverflow.com/questions/tagged/amp-html) tag at Stack Overflow.
+      - Questions about Google Search can be asked at Google's [Help Community](https://goo.gl/utQ1KZ).
+- type: textarea
+  id: description
+  attributes:
+    label: Description
+    description: Briefly describe the feature request.
+    placeholder: Provide a clear and concise description of the new feature or change to an existing feature you'd like to see.
+  validations:
+    required: true
+- type: textarea
+  id: alternatives_considered
+  attributes:
+    label: Alternatives Considered
+    description: Describe any alternatives you've considered.
+    placeholder: Provide details around any alternative solutions or features you've considered.
+  validations:
+    required: true
+- type: textarea
+  id: additional_context
+  attributes:
+    label: Additional Context
+    description: Add any other context here.
+    placeholder: Add any other context about your feature request here. E.g. paste a screenshot, or provide a link.