build(package): upgrade standard-version to 6

The previous downgrade to 5 wasn't necessary. However, I cannot
upgrade to >6 because this library is on 0.x.x and not a major
version.

Author: remarkablemark

package.json

@@ -59,7 +59,7 @@
     "rimraf": "^3.0.2",
     "rollup": "^2.12.1",
     "rollup-plugin-uglify": "^6.0.4",
-    "standard-version": "^5",
+    "standard-version": "6",
     "typescript": "^3.9.3"
   },
   "peerDependencies": {