feat(CI): check for trailing whitespace

Run on pull request only.
Use a shell script to check for trailing whitespace in all files.
Fail if trailing whitespace is found.

test/check-for-trailing-whitespace.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+cd .. || exit
+# print first
+grep -EHInr '( +)$' ./*
+
+var=$(grep -EHInr '( +)$' ./*)
+# then exit with fail if found
+if test -z "$var"; then
+  exit 0
+else
+  exit 1
+fi