Refactor state.notifications to fix pyodide

[philippjfr]

In pyodide the pn.state.notifications logic was creating a notifications object on render but not returning when the user accessed it.

Fixes #7230

[codecov]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.23%. Comparing base (d23dd3a) to head (ff03a99).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/state.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7235      +/-   ##
==========================================
- Coverage   82.23%   82.23%   -0.01%     
==========================================
  Files         337      337              
  Lines       50225    50228       +3     
==========================================
+ Hits        41301    41303       +2     
- Misses       8924     8925       +1     

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