GitHub: add issue templates for bugs and proposals

.github/ISSUE_TEMPLATE/bug.yml

@@ -0,0 +1,44 @@
+name: Bug Report
+description: File a bug report
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        A bug is when something works differently than it is expected to.
+  - type: checkboxes
+    id: searched
+    attributes:
+      label: Remember to search before filing a new report
+      options:
+        - label: I searched for this bug and did not find it in the issue tracker, and furthermore, the title I used above will make this new bug report turn up in the search results for my query.
+          required: true
+  - type: input
+    id: version
+    attributes:
+      label: Zig Version
+      description: The output of `zig version`:
+      placeholder: "0.9.0-dev.1275+ac52e0056"
+    validations:
+      required: true
+  - type: textarea
+    id: repro
+    attributes:
+      label: Steps to Reproduce
+      description: What exactly can someone else do, in order to observe the problem that you observed?
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected Behavior
+      description: What did you expect to happen?
+    validations:
+      required: true
+  - type: textarea
+    id: actual
+    attributes:
+      label: Actual Behavior
+      description: What happened instead? Be sure to include all error messages if any.
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/proposal.yml

@@ -0,0 +1,16 @@
+name: Language Proposal
+description: Propose to improve the Zig language
+labels: ["proposal"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thank you for your interest in improving the Zig language. However, we are
+        not accepting new proposals to change the language at this time.
+  - type: checkboxes
+    id: trash
+    attributes:
+      label: Please do not file a proposal to change the language
+      options:
+        - label: After clicking this box, please navigate away from this web page.
+          required: true