Rename with_child into spawn_child

[Sigma-dev]

Objective

I find myself often confusing add_child and with_child.
Other commands all use verbs to express the action, but with deviates without adding any meaning, whereas spawn is the verb used everywhere else in bevy to describe that exact action.

Therefore I propose renaming it into spawn_child

This opens the conversation on what to do with with_children, as of now spawn_child's hover would still point to it and it wouldn't change.
I am personally for renaming it to spawn_children for the same reasons as with_child (I can add it to this PR, but I thought I'd start here)

Solution

Rename it everywhere and deprecate with_child for the next version, then remove it the version after

[Maxime-Verrier]

I don't know the talks that lead to the with_child name but EntityCommands::spawn_child doesn't imply it's a child of the entity contained in the EntityCommands.

In fact it confuses me that i got an EntityCommands out of commands::spawn and not just the command that return Self since it looks really like the Commands::spawn method

At least that's my personal opinion on it, maybe i'm too used to the current api.

[hukasu]

Builder pattern usually uses with_

https://docs.rs/bevy/latest/bevy/prelude/struct.Transform.html#method.with_translation
https://docs.rs/bevy/latest/bevy/prelude/struct.UiRect.html#method.with_top
etc