For the next person who is trying to do this, I would recommend using monaco.languages.registerDocumentSemanticTokensProvider à la this blog post and adding a couple keywords to existing languages this way.

But if you really want to extend a language definition with only syntax highlighting (which according to the article is less computationally expensive than semantic highlighting) then you'll want to import from 'monaco-editor/esm/vs/basic-languages/<lang>/<lang>.js' and use monaco.languages.setMonarchTokensProvider.

You can pretty quickly find the file you need in vscode global search via including "node_modules/monaco-editor/**/*" and searching for ".js"