Param 2 compatibility to get/set the watchers of an instance

[maximlt]

Accessing ._param_watchers in the next release of Param will emit a warning, instead now the watchers can be get/set from .param.watchers.

[codecov]

Codecov Report

Merging #5350 (f23b4c4) into main (df0b277) will decrease coverage by 0.12%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##             main    #5350      +/-   ##
==========================================
- Coverage   72.70%   72.58%   -0.12%     
==========================================
  Files         274      274              
  Lines       39843    39857      +14     
==========================================
- Hits        28968    28932      -36     
- Misses      10875    10925      +50     

Flag	Coverage Δ
unitexamples-tests	72.58% <72.72%> (-0.12%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
panel/io/document.py	48.66% <33.33%> (+0.27%)	⬆️
panel/util/__init__.py	84.80% <55.55%> (-0.97%)	⬇️
panel/config.py	68.90% <100.00%> (+0.06%)	⬆️
panel/io/embed.py	86.93% <100.00%> (-0.40%)	⬇️
panel/layout/base.py	90.49% <100.00%> (ø)
panel/pane/base.py	85.56% <100.00%> (ø)
panel/tests/pane/test_base.py	84.81% <100.00%> (-12.63%)	⬇️
panel/tests/widgets/test_base.py	98.37% <100.00%> (+0.01%)	⬆️

... and 14 files with indirect coverage changes

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