[FEATURE] Support directory upload and clearing inputs from Python for FileInput widget

[hoxbro]

Problem description

I have two requests both related to the FileInput widget.

1. Make it possible to upload directories with FileInput
   The HTML element input has the property webkitdirectory, more information here. This property makes it possible to upload a directory. Note that when uploading a directory it will give a warning in the browser for security reasons.
   

2. Clearing the inputs
   I would also like to be able to clear the inputs from the Python side. In my implementation, this is done inelegantly because the parameters are readonly (more to show it is possible).

Feature description

1. Add the webkitdirectory property in BokehJS and expose it in Bokeh (I would suggest removing webkit from the name)

2. Have a clear_input method or similar

Potential alternatives

None.

Additional information

I have made a POC of it in Panel: https://github.com/holoviz/panel/pull/6808/files

But most of it is just coping over the FileInput widgets from Bokeh with minor changes.