Add watch.sh

Run yarn lint on file changes and notify-send errors.
This means I don't have to keep running yarn lint before every
commit with Husky.

watch.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+ls *.ts* | entr yarn lint | tee >(
+  while read ln; do 
+    if echo "${ln}" | grep -q "error"; then 
+      notify-send "${ln}"
+    fi
+  done
+)