Add FileDropper widget

[philippjfr]

Adds FileDropper widget based on the FilePond library. This upload component has more modern styling, support image previews and now supports chunked uploads, making it possible to upload large files.

• Resolves Drag and Drop FileInput Widget #917, How to make FileInput more reactive? #2863
• Settle on API for value attribute
• Add docs
• Add tests

[codecov]

Codecov Report

Attention: Patch coverage is 81.45161% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 81.52%. Comparing base (ec70c62) to head (3ef749d).
Report is 2 commits behind head on main.

Files	Patch %	Lines
panel/tests/ui/widgets/test_input.py	72.50%	11 Missing ⚠️
panel/widgets/input.py	80.39%	10 Missing ⚠️
panel/models/file_dropper.py	93.93%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6826       +/-   ##
===========================================
+ Coverage   39.87%   81.52%   +41.64%     
===========================================
  Files         307      319       +12     
  Lines       44938    46895     +1957     
===========================================
+ Hits        17921    38230    +20309     
+ Misses      27017     8665    -18352     

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[philippjfr]

Thanks for the tests! They look good, could you approve and merge?