[Feature]: Change PencilBrush private fields to protected

[DinosaurDad]

CheckList

• I agree to follow this project's Code of Conduct
• I have read and followed the Contributing Guide
• I have searched and referenced existing issues, feature requests and discussions
• I am filing a FEATURE request.

Description

Obviously the FabricJS 6.x codebase is now based on classes, which is great because subclassing is now easier/cleaner than with FabricJS 5.x and earlier.

I have a subclass of PencilBrush to add a few things, and override a couple of methods that work with the private fields like _points[]. As there are no get/set accessors, please consider moving the private fields to protected to facilitate subclassing.

At this time, those fields are _points, _hasStraightLine, and oldEnd.

Thank you!

Current State

Currently not able to (cleanly) access private fields in a PencilBrush subclass.

[asturur]

Please open a PR and do it.
It seems you can take care of this by yourself