Fixes #7951

Problem

The Collapse component had two conflicting requirements that couldn't be satisfied
simultaneously:

1. Stack layout issue: When a Collapse is closed inside a Stack, it should not
   contribute to the Stack's gap spacing
2. Nested animation issue: When Collapses are nested, the parent must remain in the
   DOM for child animations to work properly

Root Cause

The core issue was the use of display: 'none' in collapsed state:

• ✅ With display: 'none': Stack gap issue is resolved, but nested animations break
• ✅ Without display: 'none': Nested animations work, but Stack gap issue occurs

Solution

Added a keepMounted prop to the Collapse component that allows explicit control over
this behavior:

Default behavior (most common case)

  <Stack>
    <Button>Toggle</Button>
    <Collapse in={isOpen}>  {/* keepMounted defaults to false */}
      <Text>Content</Text>
    </Collapse>
  </Stack>

• Uses display: 'none' when collapsed
• Collapsed element doesn't contribute to Stack spacing
• ✅ Fixes the Stack gap issue

Special case (nested animations)

  <Collapse in={show} keepMounted>  {/* keepMounted={true} */}
    <SegmentedControl />
    <Collapse in={value === 'b'}>
      Content
    </Collapse>
  </Collapse>

• Doesn't use display: 'none' when collapsed
• Parent remains in DOM for child height calculations
• ✅ Enables proper nested animations

Implementation Details

• Added keepMounted?: boolean prop to CollapseProps interface
• Modified useCollapse hook to conditionally apply display: 'none'
• Updated NestedCollapseWithControl story to demonstrate the fix