Bump pngjs from 5.0.0 to 6.0.0

Bumps [pngjs](https://github.com/lukeapage/pngjs) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/lukeapage/pngjs/releases)
- [Commits](https://github.com/lukeapage/pngjs/commits)

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

package.json

@@ -87,7 +87,7 @@
     "marked": "1.2.2",
     "mocha": "8.2.0",
     "pixelmatch": "^5.1.0",
-    "pngjs": "^5.0.0",
+    "pngjs": "^6.0.0",
     "proj4": "2.6.2",
     "puppeteer": "5.4.0",
     "rollup": "^2.1.0",