ci: add a CI workflow

Add a new github workflow named CI.

Add a job named compat, checking that and old Zig compiler will not fail
with a compiler error, but instead will print an useful error message.

ci/compat.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+# This script checks that `zig build` will return an useful error message when
+# the Zig compiler is not compatible, instead of failing due to a syntax error.
+#
+# This script should be run on an UNIX system.
+
+zig_version=$(zig version)
+
+zig build -Dn=1 -Dhealed &> /dev/null 2>&1
+zig_ret=$?
+
+if [ "$zig_ret" -eq 0 ]; then
+    printf "zig %s unexpectedly succeeded\n" "$zig_version"
+    exit 1
+fi
+
+zig_error=$(zig build -Dn=1 -Dhealed 2>&1)
+
+echo "$zig_error" | grep -q "it looks like your version of zig is too old"
+zig_ret=$?
+
+if [ "$zig_ret" -ne 0 ]; then
+    printf "zig %s is not compatible\n" "$zig_version"
+    exit 1
+fi