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 69.23077% with 8 lines in your changes missing coverage. Please review.

Project coverage is 81.79%. Comparing base (dba5062) to head (aac352c).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/layout/test_column.py	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7206      +/-   ##
==========================================
- Coverage   82.21%   81.79%   -0.42%     
==========================================
  Files         339      339              
  Lines       51343    51368      +25     
==========================================
- Hits        42212    42018     -194     
- Misses       9131     9350     +219     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[philippjfr]

Since this isn't bi-directionally synced and a one time "event" this should be implemented using a server side event. Will do that now so we can merge.