fix: configure should use the parent of the current instance, to avoid duplication

[nperez0111]

Changes Overview

Based on the approach taken by #5136 and validated for appropriate behavior with a barrage of tests.

The gist of what is happening here is that extend() will always set the new child's parent property to be this (establishing a child -> parent relationship).

When doing .configure() it is doing a .extend(). Doing this directly on an extension (i.e. one that is not already .extend'd), as it will create a child -> parent relationship with pretty much the exact same config so it ends up overwriting itself with it's same options.
But what we really want is not a child -> parent relationship, but to just overwrite some options while maintaining an effective "copy" of the child (i.e. preserving it's child -> parent relationship).

This is pretty complicated, so I'm going to add a diagram explaining the current behavior and desired behavior here:

There was a lot of digging for this one:

It should resolve:

• [Bug]: Unexpected behaviour of configured extensions #4981
• [Bug]: addProseMirrorPlugins fires twice at editor initialization #4257

It may resolve:

• [Bug]: 2.2.0-rc.7 upgrade - "Error: Adding different instances of a keyed plugin (plugin$)" -- worked in 2.1.x #4704

Implementation Approach

Someone had implemented this already: #5136
I just validated that it worked and made sense with a barrage of tests. The root behavior I was trying to fix was #4704

Testing Done

Most of the code was adding tests to make sure that I understood what the problem was and if it fixed it.

Verification Steps

Run the tests with the Extension.ts file as it was before and note the tests that it fails to pass now without it

Additional Notes

Checklist

• I have renamed my PR according to the naming conventions. (e.g. feat: Implement new feature or chore(deps): Update dependencies)
• My changes do not break the library.
• I have added tests where applicable.
• I have followed the project guidelines.
• I have fixed any lint issues.

Related Issues

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[bdbch]

Nice and easy fix. looks good to me!

[634750802]

I think it's same for Nodes and Marks?

[nperez0111]

I think it's same for Nodes and Marks?

Ah jeez, I missed that. I spent so much time trying to understand it I didn't realize it. Will follow up in another PR @634750802