feat: PR template

Author: costowell

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+## What
+
+_what the PR changes_
+
+## Why
+
+_why these changes were made_
+
+## Test Plan
+
+_how did you verify these changes did what you expected_
+
+## Env Vars
+
+_did you add, remove, or rename any environment variables_
+
+## Checklist
+
+- [ ] Tested all changes locally