Enable directory uploads with FileInput

[hoxbro]

Adding the folder upload part mentioned in #6803

Also, an option to clear the FileInput from the Python side of things was added. I don't think it is the best way to do it.

TODO:

• Add tests The logic have been moved directly into Bokeh.
• Update documentation

[codecov]

Codecov Report

Attention: Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.58%. Comparing base (4e351ed) to head (65d8761).
Report is 3 commits behind head on main.

Files	Patch %	Lines
panel/reactive.py	61.53%	5 Missing ⚠️
panel/widgets/input.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6808      +/-   ##
==========================================
+ Coverage   81.16%   81.58%   +0.41%     
==========================================
  Files         323      323              
  Lines       47626    47664      +38     
==========================================
+ Hits        38658    38889     +231     
+ Misses       8968     8775     -193     

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