Align FileDownload styling with Button

[philippjfr]

• Fix text alignment Text in FileDownload Button vs Button aligned differently; one start one center #1853
• Adds support for icon
• Fix disabled handling
• Adds support for button_style
• Update docs

[codecov]

Codecov Report

Merging #4929 (323be80) into main (80da5b4) will decrease coverage by 0.01%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##             main    #4929      +/-   ##
==========================================
- Coverage   83.44%   83.43%   -0.01%     
==========================================
  Files         270      270              
  Lines       38187    38197      +10     
==========================================
+ Hits        31865    31870       +5     
- Misses       6322     6327       +5     

Flag	Coverage Δ
ui-tests	40.89% <78.57%> (+<0.01%)	⬆️
unitexamples-tests	73.38% <64.28%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
panel/widgets/misc.py	92.59% <70.00%> (-1.90%)	⬇️
panel/models/widgets.py	100.00% <100.00%> (ø)
panel/tests/ui/widgets/test_misc.py	97.43% <100.00%> (ø)

... and 1 file with indirect coverage changes

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