Update issue templates

- Include the `cranelift` label for Cranelift bug reports
- Add a Wasmtime bug report template

.github/ISSUE_TEMPLATE/wasmtime-bug-report.md

@@ -0,0 +1,18 @@
+---
+name: Wasmtime bug report
+about: Report a bug or a crash in Wasmtime
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+Thanks for opening a bug report! Please answer the questions below
+if they're relevant and delete this text before submitting.
+
+- What are the steps to reproduce the issue?
+- What do you expect to happen? What does actually happen? Does it panic, and
+  if so, with which assertion?
+- Which Wasmtime version / commit hash / branch are you using?
+- If relevant, can you include some extra information about your environment?
+  (Rust version, operating system, architecture...)