Added property insertionPointWidth

[lukepistrol]

Added insertionPointWidth to STTextView to make it possible to change the width of the insertion point.

Test result with a width of 10pt:

var textView = STTextView()
textView.insertionPointWidth = 10.0

[krzyzanowskim]

thanks! that's a valid request. I prefer providing a customization point for the insertion point (I had it once, then removed it for simplicity) rather than adding customization attributes to STTextView itself.

I just added an API that allows customizing insertion points. Maybe I move it to delegate later, but for now, it's good how it is. The pattern is akin to UIView.layerClass or NSControl.cellClass.

It works like this:

textView.insertionPointLayerClass = WideCodeInsertionPointLayer.self

class WideCodeInsertionPointLayer: STInsertionPointLayer {
    override var insertionPointWidth: CGFloat {
        10
    }

    override var insertionPointColor: NSColor {
        NSColor.yellow
    }
}

[krzyzanowskim]

that said. I'm going to merge this PR, but it may change in the future. I'd prefer to not add more insertion point customization properties to the STTextView class itself.

[lukepistrol]

Yeah that might be a better approach after all. Really like what you've done so far!