validate markdown_linkify_tlds setting (#16485)

Prevent adding * as a value to markdown_linkify_tlds site setting

lib/validators/markdown_linkify_tlds_validator.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class MarkdownLinkifyTldsValidator
+
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(value)
+    !value.include?("*")
+  end
+
+  def error_message
+    I18n.t("site_settings.errors.markdown_linkify_tlds")
+  end
+end