Bump uglify-js from 3.13.3 to 3.13.4

Bumps [uglify-js](https://github.com/mishoo/UglifyJS) from 3.13.3 to 3.13.4.
- [Release notes](https://github.com/mishoo/UglifyJS/releases)
- [Commits](mishoo/UglifyJS@v3.13.3...v3.13.4)

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -24,7 +24,7 @@
   "devDependencies": {
     "eslint": "~7",
     "phplint": "~2",
-    "uglify-js": "3.13.3"
+    "uglify-js": "3.13.4"
   },
   "dependencies": {
     "blueimp-file-upload": "10.31.0",