Blocks: Implement block insertion functions

[Bernhard Reiter]

For #59313, we need to implement functions to insert a given parsed block into another parsed block's inner blocks, and to prepend and append to that array, respectively.

We will use those functions in combination with traverse_and_serialize_blocks (see #59327) to implement automatic insertion of hooked blocks into block templates and patterns.

[Bernhard Reiter]

In 56618:

Blocks: Implement block insertion functions.

For #59313, we need to implement functions to insert a given parsed block into another parsed block's inner blocks, and to prepend and append to that array, respectively.

We will use those functions in combination with traverse_and_serialize_blocks (see #59327) to implement automatic insertion of hooked blocks into block templates and patterns.

Props gziolo.
Fixes #59385.

[Bernhard Reiter]

In 56634:

Blocks: Revert implementation of block insertion functions.

In [56618], three functions (insert_inner_block, prepend_inner_block, and append_inner_block) were introduced. They were meant to be used for insertion of hooked blocks; however, it was discovered that the original idea wouldn't work for sibling insertion. Instead, a different approach will be taken (see #59412), and these functions are no longer needed and can thus be removed.

Reverts [56618].
See #59412, #59385, #59313.