lang: funcs: Add regexp package and match function

This adds a simple regexp match function. This will be useful for
regexp based name classification, if you're into that sort of thing.

examples/lang/regexp0.mcl

@@ -0,0 +1,10 @@
+import "fmt"
+import "regexp"
+
+# test with:
+# ./mgmt run --hostname foo.example.com --tmp-prefix lang --lang examples/lang/regexp0.mcl
+# ./mgmt run --hostname db1.example.com --tmp-prefix lang --lang examples/lang/regexp0.mcl
+print "regexp" {
+	# TODO: add a heredoc string to avoid needing to escape the \ chars
+	msg => fmt.printf("match: %t", regexp.match("^db\\d+\\.example\\.com$", $hostname)),
+}