6 files declare their use of ExtensionInterface in Markdown 8.2

AllowedHtmlManager.php in src/PluginManager/AllowedHtmlManager.php

ExtensionInterface.php in src/Plugin/Markdown/CommonMark/ExtensionInterface.php

ExtensionManager.php in src/PluginManager/ExtensionManager.php

markdown.module in ./markdown.module

Markdown module.

MarkdownParamConverter.php in src/ParamConverter/MarkdownParamConverter.php

ParserManager.php in src/PluginManager/ParserManager.php