make_spec.lua: fix migration of children nodes in create_anchors
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Currently, when migrating the children of a node to a new one, children are moved as they are iterated via `cmark_node_next`; however, when a child is moved under the new node, its next relationship with its sibling is broken, so in fact at most one child is migrated, and the rest are lost. This resulted in cases like
[Decimal numeric character
references](@)
consist of...
(note the softbreak) being eventually rendered as
<a ...>Decimal numeric character</a>
consist of...
The bug is fixed in this PR.