Regex denial of service vulnerability in codesample plugin

Summary

### Impact A regex denial of service (ReDoS) vulnerability was discovered in a dependency of the `codesample` plugin. The vulnerability allowed poorly formed ruby code samples to lock up the browser while performing syntax highlighting. This impacts users of the `codesample` plugin using TinyMCE 5.5.1 or lower.

### Patches This vulnerability has been patched in TinyMCE 5.6.0 by upgrading to a version of the dependency without the vulnerability.

### Workarounds To work around this vulnerability, either: - Upgrade to TinyMCE 5.6.0 or higher - Disable the `codesample` plugin - Disable ruby code samples using the [codesample_languages](https://www.tiny.cloud/docs/plugins/opensource/codesample/#exampleusingcodesample_languages) setting - Override the PrismJS syntax highlighter to version 1.21.0 or higher using the [codesample_global_prismjs](https://www.tiny.cloud/docs/plugins/opensource/codesample/#codesample_global_prismjs) setting

### Acknowledgements Tiny Technologies would like to thank Erik Krogh Kristensen at GitHub for discovering this vulnerability.

### References https://www.tiny.cloud/docs/release-notes/release-notes56/#securityfixes

### For more information If you have any questions or comments about this advisory: * Open an issue in the [TinyMCE repo](http://github.com/tinymce/tinymce/issues) * Email us at [infosec@tiny.cloud](mailto:infosec@tiny.cloud)