ENH: Number formatting support for excel styles

[marknsikora]

• closes No way to control display format when exporting to excel #22027
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Add number formatting support to the excel styles using a fake css entry.

number-format: 0%;

Added new tests, updated docs, and manually verified outputs using both excel output engines that support styling.

[codecov]

Codecov Report

Merging #22015 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #22015      +/-   ##
==========================================
+ Coverage      92%      92%   +<.01%     
==========================================
  Files         168      168              
  Lines       50603    50605       +2     
==========================================
+ Hits        46555    46557       +2     
  Misses       4048     4048

Flag	Coverage Δ
#multiple	90.4% <100%> (ø)	⬆️
#single	42.19% <50%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/io/formats/excel.py	97.39% <100%> (+0.01%)	⬆️

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[WillAyd]

Haven't reviewed code in detail yet but does this reference an existing issue? If not can you open and add reference in whatsnew?

[marknsikora]

Done

[WillAyd]

cc @gfyoung for input.

May just be my lack of familiarity with styling but what is the preferred way of using this from an end user perspective? .apply with a UDF? Might be worth adding an example to the style notebook to illustrate

[gfyoung]

@marknsikora : This looks good at the moment. Could you add whatsnew entry for 0.24.0 ?

[TomAugspurger]

Thanks @marknsikora .