Update issue templates (rust-lang#1882)

Add new internal template and fix labels.

Co-authored-by: Zyad Hassan <[email protected]>

Author: celinval

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,8 +1,12 @@
 ---
 name: Bug Report
 about: Create a bug report for Kani
-labels: bug
+title: ''
+labels: "[C] Bug"
+assignees: ''
+
 ---
+
 <!--
 If this is a security issue, please report it following the
 [security reporting procedure](https://github.com/model-checking/kani/security/policy).

.github/ISSUE_TEMPLATE/code---infrastructure-improvement.md

@@ -0,0 +1,13 @@
+---
+name: Code / Infrastructure Improvement
+about: 'Internal issues that track improvements that don't affect Kani users, e.g.:
+  CI, code clean up.'
+title: ''
+labels: "[C] Internal"
+assignees: ''
+
+---
+
+**Proposed change:**
+
+**Motivation:**

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,8 +1,12 @@
 ---
 name: Feature Request
-about: Create a feature request for Kani
-labels: 
+about: Request a new feature or improvement for Kani
+title: ''
+labels: "[C] Feature / Enhancement"
+assignees: ''
+
 ---
+
 <!--
 If this is a security issue, please report it following the
 [security reporting procedure](https://github.com/model-checking/kani/security/policy).