Scroll when streaming to ChatFeed and ChatStep

[philippjfr]

It's a nicer experience to follow along while it's streaming though we should see if there's a mechanism to disable this behavior.

[ahuang11]

Okay now it works like auto_scroll_limit:

1. if scroll_limit = 0, then scroll_to_latest is disabled
2. if scroll_limit is null, then scroll_to_latest will always trigger
3. if scroll limit is set to a number > 0, it'll only stick to the bottom within the limit

demoo.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 72.50000% with 11 lines in your changes missing coverage. Please review.

Project coverage is 82.20%. Comparing base (55b0634) to head (ae16d20).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/layout/test_feed.py	62.06%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7608      +/-   ##
==========================================
- Coverage   82.20%   82.20%   -0.01%     
==========================================
  Files         340      340              
  Lines       51487    51522      +35     
==========================================
+ Hits        42327    42355      +28     
- Misses       9160     9167       +7     

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