add issue template

Author: ScottyPoi

.github/ISSUE_TEMPLATE/package--ethereumjs-era.md

@@ -0,0 +1,7 @@
+---
+name: 'Package: @ethereumjs/era'
+about: Create issue for @ethereumjs/era package
+title: ''
+labels: 'package: era'
+assignees: ''
+---