Add CI job to check syntax of contrib scripts

Note that `zsh` is not installed in `ubuntu-slim` by default.

.github/scripts/check-contrib.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env sh
+set -e
+
+bash --version
+zsh --version
+
+find contrib -type f \( -name '*.sh' -o -name '*.bash' \) -exec bash --norc -n {} \;
+find contrib/zsh_compl -type f -name '_rgb*' -exec zsh -n {} \;