Add ChatMessage & ChatFeed edit functionality

[ahuang11]

Closes #5761

Useful for editing & re-triggering downstream chat events if necessary

Introduces:

• enter_pressed in ChatAreaInput because it's better than watching value change and parallels TextInput's enter_pressed

3. show_edit_icon for the new edit icon that toggles the ChatMessage between original object & ChatAreaInput; only shows if message is from user
4. edited for when an edit is submitted in ChatMessage
5. Feed watches messages for any edited event; if so, call edit_callback with contents, index, instance

def echo_callback(content, index, instance):
    return content

def edit_callback(content, index, instance):
    instance.objects[index + 1].object = content

feed = ChatFeed(edit_callback=edit_callback, callback=echo_callback, callback_user="Echo Guy")
feed.send(pn.widgets.TextInput(value="Hello World!"))
feed.show()

edit.mp4

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 12 lines in your changes missing coverage. Please review.

Project coverage is 82.24%. Comparing base (ad154df) to head (eabaf0e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/ui/chat/test_chat_interface_ui.py	60.00%	6 Missing ⚠️
panel/chat/message.py	90.00%	4 Missing ⚠️
panel/chat/feed.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #7559       +/-   ##
===========================================
+ Coverage   71.90%   82.24%   +10.33%     
===========================================
  Files         338      339        +1     
  Lines       51441    51575      +134     
===========================================
+ Hits        36990    42418     +5428     
+ Misses      14451     9157     -5294     

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