Basic `VelloLine` shape and new `vello_basic` example

[nixon-voxell]

Related to https://github.com/nixon-voxell/bevy_motiongfx/issues/3

This pull request focuses on basic line shape animation and creation + some core Vello API improvements.

• Use built check instead of should_build check
• Separation of Motion and Shape creation into 2 different folders (vello_vector & vello_motion)
• Change most mods default to private to hide some crate usage only members.
• Introduction of motion plugins.
• Addition of VelloLine (alongside with motion, bundle, and motion bundle)

[nixon-voxell]

New vello_basic example:

https://github.com/nixon-voxell/bevy_motiongfx/assets/43715558/9e2c8fe1-dedc-4471-a092-d13601a590e9