add test for yarn installation

dev-setup/nvim.sh

@@ -2,6 +2,11 @@
 
 set -e
 
+if ! command -v yarn ; then
+  echo 'nvim.sh: Error - dependency `yarn` not installed'
+  exit 5
+fi
+
 if command -v apt-get ; then
   apt-get -y install ninja-build gettext libtool libtool-bin autoconf automake cmake g++ pkg-config unzip
   cd /tmp