Do not stale issues with enhancement tag (#49)

<!-- Korbit AI PR Description Start -->
## Description by Korbit AI

### What change is being made?

Modify the GitHub Actions workflow to prevent issues with the
"enhancement" label from being marked as stale.

### Why are these changes being made?

Enhancement issues often require more discussion and time before
resolution, and marking them as stale prematurely could hinder ongoing
collaboration and idea development. This change ensures such issues
remain open for necessary discussions and contributions.

> Is this description stale? Ask me to generate a new description by
commenting `/korbit-generate-pr-description`
<!-- Korbit AI PR Description End -->

.github/workflows/issue_stale.yml

@@ -12,7 +12,7 @@ jobs:
       - uses: actions/stale@v9
         with:
           operations-per-run: 250
-          exempt-issue-labels: "in progress"
+          exempt-issue-labels: "in progress,enhancement"
           days-before-pr-stale: -1
           days-before-pr-close: -1
           days-before-issue-stale: 30