fix: applying :global for +,~ sibling combinator when slots are present. fixes #9274

[kelvinsjk]

fixes #9274

For the following,

<div>
  <slot/>
  <span>xxx</span>
</div>

the style :global(p) + span could be applied because the slot could contain the required p element.

The current code did not apply the style because siblings.size===0 during compilation. To fix it, the existing find_previous_sibling and get_possible_element_siblings functions were modified to also return whether a sibling slot has been found. We only flag a css selector as inapplicable if, in addition to the existing checks on sibling size and parent, if there are no previous sibling that are slots found.

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[changeset-bot]

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[dummdidumm]

Thank you!