chore: check for clean working directory before release

is-clean.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+if ! ( output=$(git status --porcelain) && [ -z "$output" ] ); then
+    echo "Working directory isn't clean" >&2
+    exit 1
+fi

package.json

@@ -20,7 +20,7 @@
     "frontend:test:watch": "karma start --conf karma.conf.js --single-run false",
     "frontend:build": "webpack",
     "frontend:build:watch": "webpack --watch",
-    "prerelease": "npm run test",
+    "prerelease": "./is-clean.sh && npm run test",
     "release": "standard-version -a",
     "postrelease": "git push --follow-tags origin master"
   },