Replace deprecated jQuery functions $.isFunction() & $.isNumeric()

[usmonster]

jQuery 3.3 removed the $.isFunction() & $.isNumeric() functions, which can replaced by vanilla JS.

We don't need to replicate the extra browser-specific check in $.isFunction() (i.e. some built-in functions are objects in certain old versions of IE), so we might as well drop official support for That Old Browser. Since it's a backwards-incompatible change, this should probably land in a new major version.