ImportSongs: Add plugin support

- Files with filenames that end with .taikoweb.js can be imported and run to add custom functionality to the game
- The plugin file is a javascript module script that should have a class in the default export
- Currently supported methods in the class: name (string), load, start, stop, unload (functions)
- The class can be extended from the Patch class to add automatic patching of variables and functions
- Here are some of the plugins I made: https://github.com/KatieFrogs/taiko-web-plugins

public/src/js/main.js

@@ -91,6 +91,7 @@ var scoreStorage
 var account = {}
 var gpicker
 var db
+var plugins
 
 pageEvents.add(root, ["touchstart", "touchmove", "touchend"], event => {
 	if(event.cancelable && cancelTouch && event.target.tagName !== "SELECT"){