Use the same Node version as VSCode

As recommended here [1], we want to stay in sync with the current
node version shipped with VSCode (16.13.0) [2].

For this we can add a `.nvmrc` file to alert nvm to switch to
the preferred version automatically.

It will also help prevent builds from failing when setting up the
project for the first time, as building the extension currently
fails in Node v18. [3]

[1]: #1369 (comment)
[2]: https://github.com/microsoft/vscode/blob/32d40cf44e893e87ac33ac4f08de1e5f7fe077fc/remote/.yarnrc#L2
[3]: #1373

extensions/ql-vscode/.nvmrc

@@ -0,0 +1 @@
+v16.13.0