lang: Add a resource-specific elvis operator

This allows you to omit a resource parameter programmatically, and
avoids the need of an `undef` or `nil` in our language, which would
contribute to programming errors, crashes, and overall reduced safety.

examples/lang/elvis0.mcl

@@ -0,0 +1,5 @@
+$b = true # change me to false and then try editing the file manually
+file "/tmp/mgmt-elvis" {
+	content => $b ?: "hello world\n",
+	state => "exists",
+}