Clean working folder before doing a release build (#793)

Author: cjbarth

package.json

@@ -42,7 +42,7 @@
     "prettier-check": "prettier --config .prettierrc.json --check .",
     "prettier-format": "prettier --config .prettierrc.json --write .",
     "prettier-watch": "onchange -k -p 100 \".\" -- prettier --config .prettierrc.json --write {{file}}",
-    "prerelease": "npm ci && npm test && npm run build",
+    "prerelease": "git clean -xfd && npm ci && npm test && npm run build",
     "release": "release-it",
     "test": "npm run prettier-check && npm run lint && npm run tsc && nyc mocha",
     "test-watch": "mocha --watch",