add firefox extension template

externs/webextension_polyfill/extension/GetViewsFetchPropertiesType.hx

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+package webextension_polyfill.extension;
+
+typedef GetViewsFetchPropertiesType = {
+	/**
+		The type of view to get. If omitted, returns all views (including background pages and tabs). Valid values: 'tab',
+		'popup', 'sidebar'.
+		Optional.
+	**/
+	@:optional
+	var type : ViewType;
+	/**
+		The window to restrict the search to. If omitted, returns all views.
+		Optional.
+	**/
+	@:optional
+	var windowId : Float;
+	/**
+		Find a view according to a tab id. If this field is omitted, returns all views.
+		Optional.
+	**/
+	@:optional
+	var tabId : Float;
+};

externs/webextension_polyfill/extension/Static.hx

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+package webextension_polyfill.extension;
+
+typedef Static = {
+	/**
+		Returns an array of the JavaScript 'window' objects for each of the pages running inside the current extension.
+	**/
+	function getViews(?fetchProperties:GetViewsFetchPropertiesType):Array<js.html.Window>;
+	/**
+		Returns the JavaScript 'window' object for the background page running inside the current extension.
+		Returns null if the extension has no background page.
+	**/
+	function getBackgroundPage():js.html.Window;
+	/**
+		Retrieves the state of the extension's access to Incognito-mode (as determined by the user-controlled 'Allowed in
+		Incognito' checkbox.
+	**/
+	function isAllowedIncognitoAccess():js.lib.Promise<Bool>;
+	/**
+		Retrieves the state of the extension's access to the 'file://' scheme (as determined by the user-controlled 'Allow
+		access to File URLs' checkbox.
+	**/
+	function isAllowedFileSchemeAccess():js.lib.Promise<Bool>;
+	/**
+		True for content scripts running inside incognito tabs, and for extension pages running inside an incognito process.
+		The latter only applies to extensions with 'split' incognito_behavior.
+		Optional.
+	**/
+	@:optional
+	var inIncognitoContext : Bool;
+};

externs/webextension_polyfill/extension/ViewType.hx

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+package webextension_polyfill.extension;
+
+/**
+	The type of extension view.
+**/
+typedef ViewType = String;