Update issue templates (+[Improve])

.github/ISSUE_TEMPLATE/feature_request.md

@@ -0,0 +1,17 @@
+---
+name: Feature request
+about: Suggest an idea for this project
+title: "[Improve] <relative path>: <what to do>"
+labels: improve
+assignees: ''
+
+---
+
+**Rationales**
+- Reasons to do this to `Susuwu` (the namespace used throughout https://github.com/SwuduSusuwu/SubStack and possible future name for this).
+  - Which functions (if applicable) that this will improve.
+  - Which other issues/milestones (if applicable) that this will improve (but if all this is about is an issue which issues, you should just respond to that issue).
+**Background**
+- What you know about how to do so.
+- Paths to related code on [`trunk`](https://github.com/SwuduSusuwu/SubStack/tree/trunk/) or [`preview`](https://github.com/SwuduSusuwu/SubStack/tree/preview/).
+- Paths to related public sources (such as [posts on _Arxiv_](https://arxiv.org/abs/2203.05132)).