add firefox extension template

externs/webextension_polyfill/declarativenetrequest/MatchedRule.hx

@@ -0,0 +1,18 @@
+package webextension_polyfill.declarativenetrequest;
+
+typedef MatchedRule = {
+	/**
+		A matching rule's ID.
+	**/
+	var ruleId : Float;
+	/**
+		ID of the Ruleset this rule belongs to.
+	**/
+	var rulesetId : String;
+	/**
+		ID of the extension, if this rule belongs to a different extension.
+		Optional.
+	**/
+	@:optional
+	var extensionId : String;
+};