Add enhancement proposal to list of possible issue types (#738)

Problem: In the issue lifecycle document, it does not specify enhancement proposals
as an issue type. It lumps enhancement proposals under enhancements.

Solution: Specify that enhancement proposals are their own issue type.

Author: kate-osborn

ISSUE_LIFECYCLE.md

@@ -20,9 +20,9 @@ lifecycle.)
 
    `bug`: The implementation of a feature is not correct.
 
-   `enhancement`:  An enhancement, tackling technical debt, documentation changes, or improving existing features. In cases
-   where the enhancement requires an [Enhancement Proposal](/docs/proposals/README.md), the additional
-   label `enhancement-proposal` will be added.
+   `enhancement`:  An enhancement, tackling technical debt, documentation changes, or improving existing features.
+
+   `enhancement-proposal`: Enhancements that require an [Enhancement Proposal](/docs/proposals/README.md).
 
    `question`: The owner converts the issue to a github discussion and engages the creator.