Normal zooming supports customizing zoom behavior using zoomFunctions, but the drag-to-zoom functionality that's implemented by zoomRect bypasses zoomFunctions and calls updateRange correctly.

I have a custom scale implementation that relies on zoomFunctions, so trying to enable drag-to-zoom causes problems. (Specifically, I'm breaking up the axis into viewports so that I can stack multiple series within this chart - somewhat like this example. The scales for the individual portions of the chart shouldn't be directly zoomed, because their layout is managed by a separate, controlling scale, so their zoomFunctions are no-ops.)

Zooming by rectangle appears to be a distinctly different interface than zooming by percentage and focal point, so it seems reasonable to add a zoomRectFunctions to let scales customize that behavior and cover this use case.

To go with adding stronger zoomRect functionality, it makes sense to expose the `zoomRect` function directly.

The TypeScript type definitions said that standalone functions like `zoom` were exported, but these don't actually exist.  Fix that.

And properly export zoomRect.