How do I "build" an existing VS code extension?

I'm trying to use this VS Code extension: https://github.com/digama0/mm0/tree/master/vscode-mm0

I simply opened the folder in VS Code, but in the extension.ts file I see an error "Cannot find module 'vscode'." and other errors as well..

I have Node.js, npm, Yeoman and VS Code Yeoman Generator installed.