plugin/fragment: keep blocks in order under keyed x-for - #69

Since Alpine 3.15.9, keyed x-for moves only the item element and,
for x-if, its single rendered element. The content of x-fragment, x-when
and x-match was rendered before the template and was left behind, so
reordering mixed up the blocks.

Render the content after the template, like x-if, and expose its last
node through _x_currentIfEl and _x_lastRenderedEl, so x-for treats the
whole block as the item. The shared mutation observer then restores
the content of every block in the affected parent right after its
template, following nested blocks and x-if.

The observer also handles removed nodes and the records produced by
deleting nested blocks, and works with Alpine versions that do not
expose the last rendered item of x-for or destroy x-if content.

Fixes #62

Alpine's keyed x-for does not keep nested templates of its items in
order when the list changes (alpinejs/alpine#4911). The previous changes
worked around this for templates placed directly in a block, which took
most of anchor_block: tracking the items of nested x-for lists and
intercepting their lookups. This is a limitation of Alpine itself, and
alpinejs/alpine#4912 is expected to fix it, so we no longer work around
it.

anchor_block now restores only the block's own nodes, and only when they
are out of order. Content that a nested x-if, x-for or block renders
between them is left as is. It is never lost or duplicated, but it can
end up out of order, as with nested templates in plain Alpine x-for
items.

Mark the nested template reordering cases as test.fail, so they start
failing once Alpine includes alpinejs/alpine#4912