🔧 Refactorings to clean up table-of-contents transform

[brianhawthorne]

A handful of refactorings to toc.ts module in myst-transforms:

• Extract snippets for creating List and ListItem type objects into two little helper functions.
• Extract the base cases of the two mutually recursive function pairs into separate dedicated transform functions to help make the almost exactly identical structure of those two function pairs more apparent.
• Break up the three distinct sub-parts of the main function into three separate sub-functions to make the overall functionality and control flow more apparent.
• Add a few explanatory comments.

Hopefully these changes will make this module a bit more accessible to newcomers.

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Latest commit: 6181767

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