* Added helper functions to Skeleton3D for converting transforms from bone space to global space, and vice versa.
* Updated the Skeleton3D class reference.
* Changed the icon used for bones in the Skeleton3D inspector to use BoneAttachement3D's icon.
* Changed the Skeleton3D inspector to use EditorPropertyTransform and EditorPropertyVector3 when possible.
* Placed the Transform/Matrix for each bone in a sub-section, so it is visually similar to the Node3D inspector.
Which means that reduz' beloved style which we all became used to
will now be changed automatically to remove the first empty line.
This makes us lean closer to 1TBS (the one true brace style) instead
of hybridating it with some Allman-inspired spacing.
There's still the case of braces around single-statement blocks that
needs to be addressed (but clang-format can't help with that, but
clang-tidy may if we agree about it).
Part of #33027.
Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.
Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.
