Add DatetimeSlider widget

[thuydotm]

This MR adds DatetimeSlider widget, which is mentioned in #1972. The code is mainly based on Bokeh DateSlider model and the existing Panel DateSlider widget. Huge thanks to @hoxbro for your support!

TODO:

• add tests
• add reference notebook

[thuydotm]

I’m think about the unit for step param? Looking at the existing datetime select widget, DatetimeRangeSlider, the unit is in ms. But we only display the information up to seconds with format "%d %b %Y %H:%M:%S", should it be second instead of ms?

[thuydotm]

pre-commit.ci autofix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.23%. Comparing base (4e146af) to head (7398483).
Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7374      +/-   ##
==========================================
+ Coverage   81.42%   82.23%   +0.81%     
==========================================
  Files         337      338       +1     
  Lines       50514    50598      +84     
==========================================
+ Hits        41131    41611     +480     
+ Misses       9383     8987     -396     

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[hoxbro]

I have left some small comments, but this PR seems solid overall.

[thuydotm]

@hoxbro thanks for reviewing! I've pushed the changes as suggested.

[philippjfr]

Thanks @thuydotm!!