Add scroll_index to Column

[ahuang11]

Closes #6083

import panel as pn

pn.extension()

def scroll_to_top(event):
    column.scroll_index = 100


column = pn.Column(
    *list(range(1000)),
    height=300,
    sizing_mode="fixed",
    scroll=True,
)
btn = pn.widgets.Button(name="Scroll to top", width=200, button_type="primary", on_click=scroll_to_top)
pn.Row(column, btn).servable()

(poorly named btn; should be scroll to index 100)

Screen.Recording.2024-08-29.at.12.46.16.PM.mov

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.24%. Comparing base (647db1a) to head (b712cfc).

Files with missing lines	Patch %	Lines
panel/tests/ui/layout/test_column.py	30.76%	9 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7206   +/-   ##
=======================================
  Coverage   82.24%   82.24%           
=======================================
  Files         331      331           
  Lines       49420    49438   +18     
=======================================
+ Hits        40647    40662   +15     
- Misses       8773     8776    +3     

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