Make customCellTemplate a functional react component

[DhenPadilla]

Firstly, thanks for the library! We've recently adopted this and it's pretty good!

We moved a specific component from a @tanstack/react-table implementation to reactgrid, as we needed more of a spreadsheet-like implementation of data and data-mutability.

There is one thing that currently trumps reactgrid though; and that's the customizability of the cells;

Ideally, we could create our own cells easily in functional react and right now this is quite difficult with the template.
I suppose this is due to styling specific to reactgrid cells, but there is definitely an adoption 10x'er that I see could really accelerate this and it's to allow us to extend the table/grid with functional react components as 'type's.

I imagine something similar to passing a prop into the ReactGrid component (e.g customCells: DOMNode), with DefaultCell base props.

Just a thought because right now it's super hard to customize things!!
Could also see if there's a way to PR this for you

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Again, great work here! Would love to see how this moves forward ⚡