TouchBarScrubber does not expose layout options

[pelmers]

In #11038 dynamic width calculation was added to the scrubber touch bar item layout, however there is no way to specify the itemSpacing and itemSize (https://developer.apple.com/documentation/appkit/nsscrubberflowlayout?language=objc) for entries in the scrubber list, which are necessary to emulate the appearance of the TouchBarSegmentedControl class.

Desired: (TouchBarSegmentedControl)

Current appearance: (Scrubber)

Request: Likely the simplest solution would be to add optional itemSpacing and itemSize properties to the TouchBarScrubber constructor options and use those in sizeForItemAtIndex if given (https://github.com/dashersw/electron/blob/master/atom/browser/ui/cocoa/atom_touch_bar.mm#L670).

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[MarshallOfSound]

@pelmers This sounds like a reasonable enhancement, is this something you'd be willing to take a shot at? Happy to help walk you through the Electron codebase if you want to give it a go