Allow borderless SearchBox.ExpandButton

[r100-stack]

Changes

Goal of this PR is to allow a borderless SearchBox.ExpandButton during the collapsed state. This should be supported since SearchBox.ExpandButton is essentially an IconButton. However, it doesn't happen so far even with adding styleType="borderless" to SearchBox.ExpandButton since the border that on the container is never removed.

I first thought to add a new styleType prop to CollapsedState. When this prop is borderless, it sets a data attribute in iui-expandable-searchbox. Presence of this attribute will remove its border and the background. However, to decrease the complexity of the API, @mayank99 suggested to try to avoid adding a new prop.

The current approach I'm using is to directly hide the border and background on iui-expandable-searchbox when it's collapsed. No visual change should be perceived as the same background and border now come from the ExpandButton instead.

Collapsed state (before and after are the same)

Users using the SearchBox in ways that we haven't demoed in our examples may notice some change after this PR. However, after discussing with @mayank99, we concluded that those custom cases are uncommon and also unlikely to be useful for users.

Testing

Added a story and thus a visual test. Since this PR mainly adds a visual change using css styles, I felt no unit or e2e tests were necessary.

Docs

• Added one minor changeset each for @itwin/itwinui-css and @itwin/itwinui-react.
• Added docs and an accompanying demo.

[mayank99]

LGTM