Introduce templates for specific issue types (#8842)

This replaces the single github issue-reporting template with templates for the
following types of issues:

* Build issues
* Spack command errors
* Feature request

Each template includes suggestions which are generally useful for the type of
issue encountered.

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,27 @@
+---
+name: Feature request 
+about: Suggest adding a feature that is not yet in Spack  
+
+---
+
+*Please add a concise summary of your suggestion here.*
+
+### Rationale
+
+*Is your feature request related to a problem? Please describe it!*
+
+### Description
+
+*Describe the solution you'd like and the alternatives you have considered.*
+
+
+### Additional information
+*Add any other context about the feature request here.*
+
+
+-----
+
+
+If you want to ask a question about the tool (how to use it, what it can currently do, etc.), try the `#general` channel on our Slack first. We have a welcoming community and chances are you'll get your reply faster and without opening an issue.
+
+Other than that, thanks for taking the time to contribute to Spack!