build(npm): add NVM version file

The `.nvmrc` file specifies the version of Node.js that the repository's
Node.js-based tooling has been designed to be compatible with.

Users of NVM may want to run `nvm use` to install this version
automatically.

Change-Id: Ied90c51d8d1e5b43f2ca4de08a58bc782d9ae4e6
Signed-off-by: Chris Kay <chris.kay@arm.com>