add firefox extension template

externs/webextension_polyfill/permissions/AnyPermissions.hx

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+package webextension_polyfill.permissions;
+
+typedef AnyPermissions = {
+	/**
+		Optional.
+	**/
+	@:optional
+	var permissions : Array<String>;
+	/**
+		Optional.
+	**/
+	@:optional
+	var origins : Array<String>;
+};

externs/webextension_polyfill/permissions/Permissions.hx

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+package webextension_polyfill.permissions;
+
+typedef Permissions = {
+	/**
+		Optional.
+	**/
+	@:optional
+	var permissions : Array<webextension_polyfill.manifest.OptionalPermission>;
+	/**
+		Optional.
+	**/
+	@:optional
+	var origins : Array<String>;
+};

externs/webextension_polyfill/permissions/Static.hx

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+package webextension_polyfill.permissions;
+
+typedef Static = {
+	/**
+		Get a list of all the extension's permissions.
+	**/
+	function getAll():js.lib.Promise<AnyPermissions>;
+	/**
+		Check if the extension has the given permissions.
+	**/
+	function contains(permissions:AnyPermissions):js.lib.Promise<Bool>;
+	/**
+		Request the given permissions.
+	**/
+	function request(permissions:Permissions):js.lib.Promise<Bool>;
+	/**
+		Relinquish the given permissions.
+	**/
+	function remove(permissions:Permissions):js.lib.Promise<Bool>;
+	/**
+		Fired when the extension acquires new permissions.
+	**/
+	var onAdded : webextension_polyfill.events.Event<(permissions:Permissions) -> Void>;
+	/**
+		Fired when permissions are removed from the extension.
+	**/
+	var onRemoved : webextension_polyfill.events.Event<(permissions:Permissions) -> Void>;
+};