Update issue templates (rust-lang#1887)

I created a new template using the GitHub interface to see if that makes the template show up. I used a different name since the title reflects on the name of the file.

Author: celinval

.github/ISSUE_TEMPLATE/dev-improvement.md

@@ -0,0 +1,13 @@
+---
+name: Dev Improvement
+about: 'Internal issues that track improvements that don''t affect Kani users. E.g.:
+  CI, refactoring.'
+title: ''
+labels: "[C] Internal"
+assignees: ''
+
+---
+
+**Proposed change:**
+
+**Motivation:**