add firefox extension template

externs/webextension_polyfill/userscripts/RegisteredUserScript.hx

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+package webextension_polyfill.userscripts;
+
+/**
+	An object that represents a user script registered programmatically
+**/
+typedef RegisteredUserScript = {
+	/**
+		Unregister a user script registered programmatically
+	**/
+	function unregister():Void;
+};

externs/webextension_polyfill/userscripts/Static.hx

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+package webextension_polyfill.userscripts;
+
+typedef Static = {
+	/**
+		Register a user script programmatically given its $(ref:userScripts.UserScriptOptions),
+		and resolves to a $(ref:userScripts.RegisteredUserScript) instance
+	**/
+	function register(userScriptOptions:UserScriptOptions):Void;
+};

externs/webextension_polyfill/userscripts/UserScriptOptions.hx

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+package webextension_polyfill.userscripts;
+
+/**
+	Details of a user script
+**/
+typedef UserScriptOptions = {
+	/**
+		The list of JS files to inject
+	**/
+	var js : Array<webextension_polyfill.extensiontypes.ExtensionFileOrCode>;
+	/**
+		An opaque user script metadata value
+		Optional.
+	**/
+	@:optional
+	var scriptMetadata : webextension_polyfill.extensiontypes.PlainJSONValue;
+	var matches : Array<String>;
+	/**
+		Optional.
+	**/
+	@:optional
+	var excludeMatches : Array<String>;
+	/**
+		Optional.
+	**/
+	@:optional
+	var includeGlobs : Array<String>;
+	/**
+		Optional.
+	**/
+	@:optional
+	var excludeGlobs : Array<String>;
+	/**
+		If allFrames is <code>true</code>, implies that the JavaScript should be injected into all frames of current page.
+		By default, it's <code>false</code> and is only injected into the top frame.
+		Optional.
+	**/
+	@:optional
+	var allFrames : Bool;
+	/**
+		If matchAboutBlank is true, then the code is also injected in about:blank and about:srcdoc frames if your extension has
+		access to its parent document. Code cannot be inserted in top-level about:-frames. By default it is <code>false</code>.
+		Optional.
+	**/
+	@:optional
+	var matchAboutBlank : Bool;
+	/**
+		The soonest that the JavaScript will be injected into the tab. Defaults to "document_idle".
+		Optional.
+	**/
+	@:optional
+	var runAt : webextension_polyfill.extensiontypes.RunAt;
+	/**
+		limit the set of matched tabs to those that belong to the given cookie store id
+		Optional.
+	**/
+	@:optional
+	var cookieStoreId : ts.AnyOf2<String, Array<String>>;
+};