add missing externs to repo

externs/webextension_polyfill/sessions/Device.hx

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+package webextension_polyfill.sessions;
+
+typedef Device = {
+	var info : String;
+	/**
+		The name of the foreign device.
+	**/
+	var deviceName : String;
+	/**
+		A list of open window sessions for the foreign device, sorted from most recently to least recently modified session.
+	**/
+	var sessions : Array<Session>;
+};

externs/webextension_polyfill/sessions/Filter.hx

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+package webextension_polyfill.sessions;
+
+typedef Filter = {
+	/**
+		The maximum number of entries to be fetched in the requested list. Omit this parameter to fetch the maximum number of
+		entries ($(ref:sessions.MAX_SESSION_RESULTS)).
+		Optional.
+	**/
+	@:optional
+	var maxResults : Float;
+};

externs/webextension_polyfill/sessions/Session.hx

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+package webextension_polyfill.sessions;
+
+typedef Session = {
+	/**
+		The time when the window or tab was closed or modified, represented in milliseconds since the epoch.
+	**/
+	var lastModified : Float;
+	/**
+		The $(ref:tabs.Tab), if this entry describes a tab. Either this or $(ref:sessions.Session.window) will be set.
+		Optional.
+	**/
+	@:optional
+	var tab : webextension_polyfill.tabs.Tab;
+	/**
+		The $(ref:windows.Window), if this entry describes a window. Either this or $(ref:sessions.Session.tab) will be set.
+		Optional.
+	**/
+	@:optional
+	var window : webextension_polyfill.windows.Window;
+};

externs/webextension_polyfill/sessions/Static.hx

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+package webextension_polyfill.sessions;
+
+typedef Static = {
+	/**
+		Forget a recently closed tab.
+	**/
+	function forgetClosedTab(windowId:Float, sessionId:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Forget a recently closed window.
+	**/
+	function forgetClosedWindow(sessionId:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Gets the list of recently closed tabs and/or windows.
+	**/
+	function getRecentlyClosed(?filter:Filter):js.lib.Promise<Array<Session>>;
+	/**
+		Reopens a $(ref:windows.Window) or $(ref:tabs.Tab), with an optional callback to run when the entry has been restored.
+	**/
+	function restore(?sessionId:String):js.lib.Promise<Session>;
+	/**
+		Set a key/value pair on a given tab.
+	**/
+	function setTabValue(tabId:Float, key:String, value:Dynamic):js.lib.Promise<ts.Undefined>;
+	/**
+		Retrieve a value that was set for a given key on a given tab.
+	**/
+	function getTabValue(tabId:Float, key:String):js.lib.Promise<Dynamic>;
+	/**
+		Remove a key/value pair that was set on a given tab.
+	**/
+	function removeTabValue(tabId:Float, key:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Set a key/value pair on a given window.
+	**/
+	function setWindowValue(windowId:Float, key:String, value:Dynamic):js.lib.Promise<ts.Undefined>;
+	/**
+		Retrieve a value that was set for a given key on a given window.
+	**/
+	function getWindowValue(windowId:Float, key:String):js.lib.Promise<Dynamic>;
+	/**
+		Remove a key/value pair that was set on a given window.
+	**/
+	function removeWindowValue(windowId:Float, key:String):js.lib.Promise<ts.Undefined>;
+	/**
+		Fired when recently closed tabs and/or windows are changed. This event does not monitor synced sessions changes.
+	**/
+	var onChanged : webextension_polyfill.events.Event<() -> Void>;
+	/**
+		The maximum number of $(ref:sessions.Session) that will be included in a requested list.
+	**/
+	var MAX_SESSION_RESULTS : Int;
+};