Expose ICellArray handle to extensions

[Kenzzer]

Resolves #720

Throughout my time using Sourcemod, I've come across multiple situations where being able to create a cell array handle and reading one would have been quite useful when designing natives for my extensions. Instead I've had to resort into creating a new handle type that mimics the behaviour of ArrayList, which is a source of many problems. It's extra work to put in place and unnecessary friction for plugin authors that need to read more documentation so they know how to loop through the pseudo-array like object. The current situation also adds an extra layer of complexity should the plugin author wants to make use of the ArrayList natives as they'd need to transpose the whole content from the extension's arraylike handle, to a new handle (arraylist) they created.

That feeling seems to have been echoed by other extensions authors in the past (see the linked issue), and still to this day I hear that desire from multiple places. I believe this is a justified addition to the extension API.