[mlir-vscode] Explicitly set the return type for didOpen

[rriddle]

Summary

In the newer versions of the language client, this explicitly expects a
Promise<void> return type, otherwise it errors out.

Fixes #56297

mlir/utils/vscode/src/mlirContext.ts

async startLanguageClient(workspaceFolder: vscode.WorkspaceFolder,
     // folders. Workspace folder language servers can properly filter out
     // documents not within the folder, but we can't effectively filter for
     // documents outside of the workspace. To support this, and avoid having two
     // servers targeting the same set of files, we use middleware to inject the
     // dynamic logic for checking if a document is in the workspace.
     let middleware = {};
     if (!workspaceFolder) {
       middleware = {
-        didOpen : (document, next) => {
+        didOpen : (document, next) : Promise<void> => {
           if (!vscode.workspace.getWorkspaceFolder(document.uri)) {
-            next(document);
+            return next(document);
           }
+          return Promise.resolve();
         }
       };
     }
 
     // Configure the client options.
     const clientOptions: vscodelc.LanguageClientOptions = {
       documentSelector : [
         {scheme : 'file', language : languageName, pattern : selectorPattern}