Retain existing config when calling configure on Marks and Extensions (attempt 2)

[nholden]

This is a second attempt at addressing #3813. My first attempt, #3822, had to be reverted because of issues described in this comment. That comment described a defaultOptions warning that I was able to reproduce in my original approach, and I confirmed it's no longer present in this approach. cc @bdbch

Currently, when you call .configure() on a Mark or Extension, the Mark or Extension that's returned doesn't retain the config from the original.

This is a problem for StarterKit. When Code is added to an editor added as a standalone extension, an inline code block is exitable. When added as part of StarterKit, an inline code block is not exitable because StarterKit calls Code.configure(), blowing away the default config.

This PR ensures that when you call .configure() on a Mark or Extension, the Mark or Extension that's returned retains the config from the original.

closes #3813

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[bdbch]

Hey @nholden - sorry for not reviewing this PR earlier. This looks promising – at least I didn't encounter the issues we experienced with the first draft. Do you mind we merge this and bring it into 2.2.0-rc.1 first so we can test it out with more people?

[nholden]

Sounds great. Thanks, @bdbch!

[Sembauke]

can we merge this?

[nperez0111]

This has already been resolved by #4191