Joint2D and Joint3D: update joint on NOTIFICATION_POST_ENTER_TREE

[rburing]

In contrast with updating only on NOTIFICATION_READY, this allows reparenting, etc.

Fixes #31711 and fixes #41398 in master.

Also fixes a related issue (in a separate commit): makes set_exclude_nodes_from_collision respect signal connections.

[akien-mga]

Is there a specific reason why it needs to be in POST_ENTER_TREE instead of ENTER_TREE? Not that it's wrong but we usually use the latter so unless the post is important here for timing, it could use ENTER_TREE for consistency.

[rburing]

This is to guarantee that all the possible node setups which were previously admissible still remain admissible, e.g. with a body being a child of the joint. Using ENTER_TREE would limit the possible node setups, which could be a future design decision (but would need the addition of an error and documentation); the current PR preserves the status quo of admissible node setups.

[akien-mga]

Thanks!