Implement Placeholder pane

[ahuang11]

Philipp and I were discussing #6617 and thought it'd be useful to have a placeholder pane to easily replace the contents with any object.

Previously:

column = pn.Column("Hello")
column

column.objects = [pn.widgets.TextInput(value="Hello again!")]

Now:

placeholder = pn.pane.Placeholder("⏳ Idle")
placeholder

placeholder.object = pn.widgets.TextInput(value="Hello again!")

Additionally, it can be used as a context manager to replace objects and revert back to the original.

Screen.Recording.2024-04-24.at.12.49.53.PM.mov

This could potentially be used to refactor ChatMessage too.

[codecov]

Codecov Report

Attention: Patch coverage is 97.91667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 70.98%. Comparing base (cdcd074) to head (73b1260).
Report is 48 commits behind head on main.

Files	Patch %	Lines
panel/pane/placeholder.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6790       +/-   ##
===========================================
+ Coverage   22.10%   70.98%   +48.87%     
===========================================
  Files         314      317        +3     
  Lines       46326    46692      +366     
===========================================
+ Hits        10240    33142    +22902     
+ Misses      36086    13550    -22536     

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[ahuang11]

Unrelated test failures

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[1], line 1
----> 1 from ipyleaflet import Map, VideoOverlay
      3 m = Map(center=(25, -115), zoom=4)
      5 video = VideoOverlay(
      6     url="https://www.mapbox.com/bites/00188/patricia_nasa.webm",
      7     bounds=((13, -130), (32, -100))
      8 )

ModuleNotFoundError: No module named 'ipyleaflet'

[philippjfr]

Added an .update method since it looks a little cleaner imo and can more easily be used in a callback or pipeline.