Update issue templates (highlightjs#2381)

Add some issue templates to see if they are useful.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,20 @@
+---
+name: Feature request
+about: Suggest a new idea or feature...
+title: "[Request] ..."
+labels: enhancement
+assignees: ''
+
+---
+
+**Is your request related to a specific problem you're having?**
+A clear and concise description of the problem itself. Ex. I'm always frustrated when [...]
+
+**The solution you'd prefer / feature you'd like to see added...**
+A clear and concise description of how you imagine we might solve this (if you have any ideas).
+
+**Any alternative solutions you considered...**
+A clear and concise description of any alternative solutions or features you may have considered.
+
+**Additional context...**
+Add any other context or screenshots relating to the feature request here.