fix(package): update tsutils to version 3.15.0

Closes #88

Author: greenkeeper[bot]

package.json

@@ -36,7 +36,7 @@
   "dependencies": {
     "@fimbul/bifrost": "^0.17.0",
     "tslib": "^1.7.1",
-    "tsutils": "^2.29.0"
+    "tsutils": "^3.15.0"
   },
   "devDependencies": {
     "@fimbul/mithotyn": "^0.17.0",