Add issue templates

As proposed in whatwg/meta#296.

.github/ISSUE_TEMPLATE/0-new-issue.yml.template

@@ -0,0 +1,17 @@
+name: New issue
+description: File a new issue against the @@h1@@ Standard.
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Before filling out this form, please familiarize yourself with the [Code of Conduct](https://whatwg.org/code-of-conduct). You might also find the [FAQ](https://whatwg.org/faq) and [Working Mode](https://whatwg.org/working-mode) useful.
+
+        If at any point you have questions, please reach out to us on [Chat](https://whatwg.org/chat).
+  - type: textarea
+    attributes:
+      label: "What is the issue with the @@h1@@ Standard?"
+    validations:
+      required: true
+  - type: markdown
+    attributes:
+      value: "Thank you for taking the time to improve the @@h1@@ Standard!"

.github/ISSUE_TEMPLATE/1-new-feature.yml.template

@@ -0,0 +1,27 @@
+name: New feature
+description: Request a new feature in the @@h1@@ Standard.
+labels: ["addition/proposal", "needs implementer interest"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Before filling out this form, please familiarize yourself with the [Code of Conduct](https://whatwg.org/code-of-conduct), [FAQ](https://whatwg.org/faq), and [Working Mode](https://whatwg.org/working-mode). They help with setting expectations and making sure you know what is required. The FAQ ["How should I go about proposing new features to WHATWG standards?"](https://whatwg.org/faq#adding-new-features) is especially relevant.
+
+        If at any point you have questions, please reach out to us on [Chat](https://whatwg.org/chat).
+  - type: textarea
+    attributes:
+      label: "What problem are you trying to solve?"
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: "What solutions exist today?"
+  - type: textarea
+    attributes:
+      label: "How would you solve it?"
+  - type: textarea
+    attributes:
+      label: "Anything else?"
+  - type: markdown
+    attributes:
+      value: "Thank you for taking the time to improve the @@h1@@ Standard!"

.github/ISSUE_TEMPLATE/config.yml.template

@@ -0,0 +1,8 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Chat
+    url: https://whatwg.org/chat
+    about: Please do reach out with questions and feedback!
+  - name: Stack Overflow
+    url: https://stackoverflow.com/
+    about: If you're having trouble building a web page, this is not the right repository. Consider asking your question on Stack Overflow instead.