Update Xrm.Portal.js

[sarjanwebdev]

Refactored the portal control system by converting legacy function constructors into ES6 classes with a clear inheritance hierarchy—BaseControl and GenericControl now serve as the foundation for all specialized controls (Lookup, Subgrid, QuickView, etc. and new Multiselect). Enhanced documentation with JSDoc comments and organized code using #region blocks, while modernizing syntax, improving error handling, and preserving full backward compatibility within the Xrm.Portal framework.