Move to using multiple issue and PR templates (#1584)

.github/PULL_REQUEST_TEMPLATE/new_feature.md

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+# New feature addition
+
+<!--
+To be used for PRs which add a new feature
+Examples of this include new APIs, new core cogs, etc.
+-->
+
+#### What type of feature is this?
+
+<!-- To check a box, replace the space between the [] with a x -->
+
+- [ ] New core cog
+- [ ] New API
+- [ ] Other
+
+#### Describe the feature
+
+<!--
+If you are adding a cog, describe its commands in detail (functionality, usage restrictions, etc).
+If the new feature introduces new requirements, please try to explain why they are necessary.
+-->