Add type stubs for functions in matplotlib.dates

[dstansby]

PR summary

This is a start at tackling #26942. To keep the PR manageable, I've started with just the functions at the top of the file.

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[QuLogic]

Note, this file was already done by a new contributor in #30126; would you be interested in tackling the final comments there instead?

[dstansby]

Note, this file was already done by a new contributor in #30126; would you be interested in tackling the final comments there instead?

It looks like this PR is stalled, but I'd be happy to take it over. Do you have a preference on whether this PR should be merged or closed before I finish of the work in #30126? I'd have a soft preference to merge this since it's smaller and already has one approval.