feat(fillability): Don't allow shrinking of non-fill items in fillable containers

[gadenbuie]

Fixes #370

As currently implemented, both fill items and non-fill items will compete for vertical space in the fillable container's flex context. In the screenshot below (derived from the example in #370), the fillable container has a height of 300px and it contains two children with a set height of 250px (so together they would otherwise overflow the container were it not fillable [flex]). The first child is a fill item and the second child is not. In the current implementation, they equally share the vertical space in the fillable container, thanks to browser defaults of flex-shrink: 1 allowing overflowing items to shrink in display: flex.

This PR changes non-fill item children of fillable containers to have flex-shrink: 0, meaning that they'll use their initial height. The non-fill item in this scenario now has a height of 250px and the fill item shrinks as expected to fill the remaining space.

In general, users should set min-height and max-height to constrain fill items. This PR doesn't interfere with that advice, but it does mean that setting height is now the preferred method for setting the height of a non-fill item. Here's another example, similar to the one above, where the fillable container contains a fill item with min-height of 35px, a non-fill item with height: 250px and a fill item without an intrinsic height. Currently, all three share equal space, but in this PR the first fill item uses 35px, the non-fill item uses 250px and the unbounded fill item uses the remaining 15 pixels.

Another weird issue this PR avoids is that currently, because the non-fill item is allowed to shrink, its final height is proportional to the overall height of all of the flex children before shrinking. In other words, if we request a height of 100px for the non-fill item, it currently will have a final height of 50px (100 / (250 + 100 + 250) * 300). After this PR, the non-fill item has the requested 100px height and the remaining 200 pixels are shared between the fill items.

[jcheng5]

LGTM. Sorry, I misunderstood and thought this was going to be more complicated than it is.