Add queue_free_children and free_children

[shoyguer]

Closes godotengine/godot-proposals#764.
Related to #94163.
Co-authored-by: @radiantgurl

I added:

• Add free_children() to Node class
• Add queue_free_children() to Node class

Optimization test

Testing of speed using 1000 children iterating for 500 times.

free_children() (in tree)
  Manual loop:     386.574 ms
  Native loop:       148.931 ms
  Speedup:         2.60x

queue_free_children() (in tree)
  Manual loop:     92.189 ms
  Native loop:       32.323 ms
  Speedup:         2.85x

free_children() (not in tree)
  Manual loop:     238.888 ms
  Native loop:       137.957 ms
  Speedup:         1.73x

queue_free_children() (not in tree)
  Manual loop:     92.856 ms
  Native loop:       36.007 ms
  Speedup:         2.58x

As you can see, the free_children() on the "not in tree" gain is slower, and it is because the standalone path skips most SceneTree overhead already, runtime then leans on per-child memdelete cost, and relative gain ends lower than in-tree path.

PC build for testing:

Windows 11
AMD Ryzen 5 5600
AMD RX 6750XT 12GB
48GB RAM 3200MHz
SSD M.2 512GB

[shoyguer]

@AThousandShips , done