fix(#7913): Prevent erroneous warning when using <slot> inside slot-scope

[mpbarlow]

Because slotNodes inside a slot-scope context are already set to _rendered = true after initial
render, the warning for duplicate slot presence always fires when a slot-scope prop change triggers
a re-render. With this change, the compiler tracks whether any slot-scoped elements have been
encountered yet at the point the slot is compiled. If so, the direct ancestors of the slot are checked
for slot-scope presence, and if one is found, the warning is supressed.

This is admittedly not a perfect solution, as within a slot-scope context the warning now does not fire even when there are duplicate slots, but I couldn't find a good way to get around that.

fix #7913

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[yyx990803]

Thanks for the PR! I managed to completely remove the "no duplicated slots" restrictions in 530ca1b, so the warning is no longer necessary.