restore old ExpandableBlock api

[mayank99]

Changes

undid the breaking change from #1354 and brought back title/caption/endIcon props while also supporting the new subcomponents added in that PR.

renamed ExpandableBlock.Header to ExpandableBlock.Trigger as it makes more sense.

these two snippets are now equivalent:

<ExpandableBlock title='click me' caption='or not' endIcon={<svg>...</svg>}>
  content in block
</ExpandableBlock>

<ExpandableBlock.Wrapper>
  <ExpandableBlock.Trigger>
    <ExpandableBlock.ExpandIcon />
    <ExpandableBlock.LabelArea>
      <ExpandableBlock.Title>click me</ExpandableBlock.Title>
      <ExpandableBlock.Caption>or not</ExpandableBlock.Caption>
    </ExpandableBlock.LabelArea>
    <ExpandableBlock.EndIcon><svg>...</svg></ExpandableBlock.EndIcon>
  </ExpandableBlock.Trigger>
  <ExpandableBlock.Content>content in block</ExpandableBlock.Content>
</ExpandableBlock.Wrapper>

Testing

updated one story and added one unit test.

Docs

updated old changeset. updated documentation page.

will update migration guide too.

[r100-stack]

Having some website examples that still use the old API would be good.

[mayank99]

Having some website examples that still use the old API would be good.

Updated. 3e65703