Blocks: Implement `insert_hooked_blocks()`

[Bernhard Reiter]

For #59313, we need a function that will plug into parsed block tree traversal and serialization ([56620]). For each block traversed, it will obtain the list of all hooked blocks that use it as their anchor block (get_hooked_blocks, [56610]), and insert those hooked blocks into the parsed block tree (insert_inner_block, prepend_inner_block, append_inner_block, [56618]).

We'll tentatively call it insert_hooked_blocks.

[Bernhard Reiter]

No longer required per [56649]. Closing as wontfix.