type: Return Self for .servable

[hoxbro]

I noticed this code will confuse the LSP.

import panel as pn

button = pn.widgets.Button().servable()
button.on_click()

Before:

After:

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.21%. Comparing base (7afcc90) to head (7cc71d2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/template/base.py	0.00%	1 Missing ⚠️
panel/viewable.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7530      +/-   ##
==========================================
- Coverage   82.21%   82.21%   -0.01%     
==========================================
  Files         339      339              
  Lines       51319    51321       +2     
==========================================
  Hits        42193    42193              
- Misses       9126     9128       +2     

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