feat: add Knip's ignoreExportsUsedInFile (#491)

## PR Checklist

- [x] Addresses an existing open issue: fixes #490
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Brings in the option as described.

Author: JoshuaKGoldberg

knip.jsonc

@@ -8,5 +8,9 @@
 	],
 	"ignoreBinaries": ["dedupe", "gh"],
 	"ignoreDependencies": ["c8"],
+	"ignoreExportsUsedInFile": {
+		"interface": true,
+		"type": true
+	},
 	"project": ["src/**/*.ts!", "script/**/*.js"]
 }