add firefox extension template

externs/webextension_polyfill/networkstatus/NetworkLinkInfo.hx

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+package webextension_polyfill.networkstatus;
+
+typedef NetworkLinkInfo = {
+	/**
+		Status of the network link, if "unknown" then link is usually assumed to be "up"
+	**/
+	var status : NetworkLinkInfoStatusEnum;
+	/**
+		If known, the type of network connection that is avialable.
+	**/
+	var type : NetworkLinkInfoTypeEnum;
+	/**
+		If known, the network id or name.
+		Optional.
+	**/
+	@:optional
+	var id : String;
+};

externs/webextension_polyfill/networkstatus/NetworkLinkInfoStatusEnum.hx

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+package webextension_polyfill.networkstatus;
+
+/**
+	Status of the network link, if "unknown" then link is usually assumed to be "up"
+**/
+typedef NetworkLinkInfoStatusEnum = String;

externs/webextension_polyfill/networkstatus/NetworkLinkInfoTypeEnum.hx

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+package webextension_polyfill.networkstatus;
+
+/**
+	If known, the type of network connection that is avialable.
+**/
+typedef NetworkLinkInfoTypeEnum = String;

externs/webextension_polyfill/networkstatus/Static.hx

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+package webextension_polyfill.networkstatus;
+
+typedef Static = {
+	/**
+		Returns the $(ref:NetworkLinkInfo} of the current network connection.
+	**/
+	function getLinkInfo():Void;
+	/**
+		Fired when the network connection state changes.
+	**/
+	var onConnectionChanged : webextension_polyfill.events.Event<(details:NetworkLinkInfo) -> Void>;
+};