Since the contribution guidelines advise on a specific version of node,
it adding .nvmrc for those that use nvm will automatically let them use the
correct version of node if they do nvm use
just a small QOL thing for devs.
Co-authored-by: João Moreno <joao.moreno@microsoft.com>
As noted in [the wiki](https://github.com/Microsoft/vscode/wiki/How-to-Contribute#prerequisites) node's version should be larger than 8.12 so 8.9.2 will not work.
In fact, I think we only need to define the target version as `8` so we can have the latest version that satisfies the requirement (`>= 8.12.0, < 9.0.0`). At the time of writing it is `8.15` which works perfectly fine (install + watch + run).
