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]

We're also updating the docs to mention using `nvm` to manage node
versions.

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

CONTRIBUTING.md

@@ -31,6 +31,8 @@ Here are a few things you can do that will increase the likelihood of your pull
 
 Make sure you have installed recent versions of vscode (>= v1.52), node (>=12.16), and npm (>= 7.5.2). Earlier versions will probably work, but we no longer test against them.
 
+To automatically switch to the correct version of node, we recommend using nvm which will pick-up the node version from `.nvmrc`.
+
 ### Installing all packages
 
 From the command line, go to the directory `extensions/ql-vscode` and run

extensions/ql-vscode/.nvmrc

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