Make Polygon2D::bones property internal

[raulsntos]

According to the documentation, it sounds like this property was supposed to be internal. So I'm adding the PROPERTY_USAGE_INTERNAL flag.

• Related to Mark underscored properties as internal #94954.
• Contributes to Ignoring class methods with underscore prefix cause issue with some properties #64429.

[akien-mga]

Seems fine to me. I guess this technically breaks compat as this could be accessed from script before?

[KoBeWi]

Internal properties can still be accessed in script, they just don't show in documentation and autocompletion.

[akien-mga]

Thanks!