VS Code extension throws on launch if Q# notebook cell is open (#2044)

**Repro**

1. Create a notebook like so - with one cell with language set to "Q#"
but no `%%qsharp` magic


![image](https://github.com/user-attachments/assets/944aca1e-96f2-4660-9ada-30e6d9e50b66)

2. Save this notebook, and launch VS Code with only this notebook open

Exception:

```
2024-11-25 12:23:42.755 [error] ReferenceError: Cannot access 'defaultLanguageId' before initialization
	at updateQSharpCellLanguages (http://localhost:3000/static/devextensions/out/extension.js#vscode-extension:40620:57)
	at eval (http://localhost:3000/static/devextensions/out/extension.js#vscode-extension:40577:7)
	at Array.forEach (<anonymous>)
	at registerQSharpNotebookHandlers (http://localhost:3000/static/devextensions/out/extension.js#vscode-extension:40575:40)
	at activate (http://localhost:3000/static/devextensions/out/extension.js#vscode-extension:40979:33)
	at async F1.n (http://localhost:3000/static/build/out/vs/workbench/api/worker/extensionHostWorkerMain.js:113:4450)
	at async F1.m (http://localhost:3000/static/build/out/vs/workbench/api/worker/extensionHostWorkerMain.js:113:4413)
	at async F1.l (http://localhost:3000/static/build/out/vs/workbench/api/worker/extensionHostWorkerMain.js:113:3869)
```

--------

The bug comes down to this sort of pattern causing an exception in
JavaScript:
```js
foo(); // ReferenceError: Cannot access 'x' before initialization

let x = 1;

function foo() { x } 
```

Fix is to declare `defaultLanguageId` at the global scope so that
initialization is guaranteed before the function that closes over it is
ever called.

vscode/src/notebook.ts

@@ -11,6 +11,7 @@ import { notebookTemplate } from "./notebookTemplate.js";
 const qsharpCellMagic = "%%qsharp";
 const jupyterNotebookType = "jupyter-notebook";
 const qsharpConfigMimeType = "application/x.qsharp-config";
+let defaultLanguageId: string | undefined;
 
 /**
  * Sets up handlers to detect Q# code cells in Jupyter notebooks and set the language to Q#.
@@ -53,8 +54,6 @@ export function registerQSharpNotebookHandlers() {
     }),
   );
 
-  let defaultLanguageId: string | undefined;
-
   function updateQSharpCellLanguages(cells: vscode.NotebookCell[]) {
     for (const cell of cells) {
       // If this is a code cell that starts with %%qsharp, and language wasn't already set to Q#, set it.