Allow custom widths for `PageLayout.Pane`

[iansan5653]

PageLayout.Pane only allows three values for the width prop: "small" | "medium" | "large". The real default values of these keywords can be found here:

react/src/PageLayout/PageLayout.tsx

Line 526 in 84993e7

const defaultPaneWidth = {small: 256, medium: 296, large: 320}

These are fine when the pane is used for a vertical menu like it usually is, but there are other use-cases as well. For example, you might want to have a details/preview pane that takes up a third or more of the screen width by default. This should ideally be much wider than 320px. In this case, it would be very useful to be able to set a custom width.

For a resizable pane, we also need to be able to set minimum and maximum widths:

width={{
 min: '400px',
 default: '550px',
 max: '700px'
}}

[mperrotti]

I see the value in making this update - especially the min/max for resizable panes. It would also be good to take a look at how this is being used and codify the most commonly used values.

@lesliecdubs - I think we can give this back to eng to implement.