add firefox extension template

externs/webextension_polyfill/normandyaddonstudy/Static.hx

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+package webextension_polyfill.normandyaddonstudy;
+
+typedef Static = {
+	/**
+		Returns a study object for the current study.
+	**/
+	function getStudy():Void;
+	/**
+		Marks the study as ended and then uninstalls the addon.
+	**/
+	function endStudy(reason:String):Void;
+	/**
+		Returns an object with metadata about the client which may be required for constructing survey URLs.
+	**/
+	function getClientMetadata():Void;
+	/**
+		Fired when a user unenrolls from a study but before the addon is uninstalled.
+	**/
+	var onUnenroll : webextension_polyfill.events.Event<(reason:String) -> Void>;
+};

externs/webextension_polyfill/normandyaddonstudy/Study.hx

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+package webextension_polyfill.normandyaddonstudy;
+
+typedef Study = {
+	/**
+		The ID of the recipe for the study.
+	**/
+	var recipeId : Float;
+	/**
+		A slug to identify the study.
+	**/
+	var slug : String;
+	/**
+		The name presented on about:studies.
+	**/
+	var userFacingName : String;
+	/**
+		The description presented on about:studies.
+	**/
+	var userFacingDescription : String;
+	/**
+		The study branch in which the user is enrolled.
+	**/
+	var branch : String;
+	/**
+		The state of the study.
+	**/
+	var active : Bool;
+	/**
+		The ID of the extension installed by the study.
+	**/
+	var addonId : String;
+	/**
+		The URL of the XPI that was downloaded and installed by the study.
+	**/
+	var addonUrl : String;
+	/**
+		The version of the extension installed by the study.
+	**/
+	var addonVersion : String;
+	/**
+		The start date for the study.
+	**/
+	var studyStartDate : webextension_polyfill.extensiontypes.DateType;
+	/**
+		The end date for the study.
+	**/
+	var studyEndDate : webextension_polyfill.extensiontypes.DateType;
+	/**
+		The record ID for the extension in Normandy server's database.
+	**/
+	var extensionApiId : Float;
+	/**
+		A hash of the extension XPI file.
+	**/
+	var extensionHash : String;
+	/**
+		The algorithm used to hash the extension XPI file.
+	**/
+	var extensionHashAlgorithm : String;
+};