feat(button): add isSpread prop

[agreatscott]

There's a design need to have a button with an end icon, where the two are split across the length of the button (think space-between). This PR attempts to implement that in the current Button component.

I don't love the implementation, and it's a bit messy, but it does the trick. Open to thoughts/comments.

Desired from Figma:

Actual storybook with this change

[akdetrick]

Biggest concerns:

• startContent, startIcon, endContent, endIcon may need different flex rules
• SplitButton will also take this prop. What does a spread splitButton look like? We probably want to pack all button internal items and make the secondary button the shrink member of the flex

I restarted the Chromatic action to take a look at the preview build

[akdetrick]

I think what we want to happen with isSpread is for the root element (the button or anchor) takes up 100% of parent width.

Expected

(we may also want to set a self-justification for the content area so it justifies start when isSpread)

Actual

[akdetrick]

Two changes requested:

Ensure the button fills parent width when isStretch

We want the entire button to have width: 100% so it fills its parent container.
It might also be nice to left align

Special handling for SplitButton

The SplitButton component takes all the same props as Button, including the new isSpread. A spread split button should look like this:

You can intercept the isSpread props to apply a 100% width to the root element of SplitButton when the prop is truthy

tl;dr

We want the layout for Button to be like this when spread (start and end content & icons in red):

For SplitButton:
Fill 100% width and let the Button size stretch