New version of "purify" declares a source map, but it's not included in Cesium source

#9241 upgrades the `purify` library which declares a source map here: https://github.com/CesiumGS/cesium/commit/4d4892b5e30bf85841effc7df93706cd62acfa38#diff-1b4d88e996ee00af5f71adfe5aafd9188412f0990b0d1d90fa58c12c1477edd3R1159

However the source map is not included in the Cesium codebase, so I'm getting a 404 warning during development:

```
DevTools failed to load SourceMap: Could not load content for .../Cesium/Source/ThirdParty/purify.es.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE
```

This is just an annoyance, nothing is broken. Thanks in advance!

cc @mramato

Start with Source/ThirdParty/purify.es.js and the purify upgrade referenced in #9241, then inspect the related source-map declaration. Confirm the corresponding map is present in the Cesium source and reload the development page to verify the DevTools 404 warning is gone.

Written by the indexing model from the issue text.