Add scroll options to permanently toggle on scrollbar

[ahuang11]

Adds option to permanently turn on scroll in direction(s), like for use in pn.Feed to prevent the layout from bouncing around if objects are added / removed.

import panel as pn

col = pn.Column(
    pn.Spacer(styles=dict(background='red'), width=200, height=200),
    pn.Spacer(styles=dict(background='green'), width=200, height=200),
    pn.Spacer(styles=dict(background='blue'), width=200, height=200),
    scroll="y", height=1000
)
col.servable()

[codecov]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (4689f9f) 82.66% compared to head (c83f297) 71.63%.

Files	Patch %	Lines
panel/layout/base.py	29.41%	12 Missing ⚠️
panel/tests/ui/layout/test_column.py	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6266       +/-   ##
===========================================
- Coverage   82.66%   71.63%   -11.04%     
===========================================
  Files         305      305               
  Lines       45429    45440       +11     
===========================================
- Hits        37552    32549     -5003     
- Misses       7877    12891     +5014     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	71.63% <17.85%> (-0.08%)	⬇️

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