Rewrite emphasis handling

[facelessuser]

• This is a complete rewrite of how emphasis handling is done.
• Drop use of multiple regex patterns in run in multiple passes and instead evaluate delimiters, nested or otherwise, and build up HTML elements.
• Try to consume tokens as much as possible until a full element is constructed (with children if any).
• If an outer set of tokens cannot be resolved, but one or more sub tokens can, render the first sub token span and cache the remaining ones for subsequent reentry and render those until the cache is exhausted.
• Two tests results were updated to match new behavior.

Description

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[facelessuser]

This is purposely a draft and is being made available for testing. More tests should be written, etc. For review.

The proposal is to first release this in Pymdown Extensions, and once vetted in the real world, make it the default approach, but I will let Python Markdown make the decision to release it before then if desired.

[waylan]

Awesome! I haven't had an opportunity to fully review the code, but the idea seems sound. The one thing that gives me pause is that we are reducing the number of inline processors to run. In itself, that is not a problem, but it may be that some third-party extensions are expecting to inject their processors between a few of the now-combined processors. I'm not certain if that will cause issues for those extensions or not. Therefore, my inclination is to consider this a backward incompatible change. Presumably the release notes should include a warning for third-party extension devs. I don't think there is any way we can gracefully make these changes through deprecations.

Interestingly, the change this relies on already was pushing the next release into at least a minor release. Adding this to the same release would be ideal from that perspective. But I also see the desire to test more thoroughly. I'm undecided about it for now.