Author here. I wrote both the language server and Muon compiler [1] from scratch. The language server reuses the compiler code, for parsing and type checking. I'm not sure which grammar you're referring to? The VS Code extension [2] does contain a grammar, but it's only used for syntax highlighting.

[1] https://github.com/nickmqb/muon/tree/master/compiler [2] https://github.com/nickmqb/vscode-muon