I have a reasonably simple extension for Visual Studio Code where I want to apply some warning, if possible, without having to implement an entire language server for it. Is
So, I was able to do this after a bunch of digging.
import { languages, Diagnostic, DiagnosticSeverity } from 'vscode';
...
let diagnosticCollection = languages.createDiagnosticCollection("stuff");
let diagnostics : Diagnostic[] = [];
...
diagnostics.push(new Diagnostic(range, message, DiagnosticSeverity.Warning));
diagnosticCollection.set(document.uri, diagnostics);