Add JSONEditor widget

[philippjfr]

Uses JSONEditor library to edit JSON in a tree view.

[MarcSkovMadsen]

Is it also for editing List and Dict parameters?

[philippjfr]

Is it also for editing List and Dict parameters?

Certainly could be.

[codecov]

Codecov Report

Merging #1974 (58814da) into master (06cb858) will decrease coverage by 0.04%.
The diff coverage is 89.36%.

@@            Coverage Diff             @@
##           master    #1974      +/-   ##
==========================================
- Coverage   82.99%   82.95%   -0.05%     
==========================================
  Files         192      193       +1     
  Lines       25481    25572      +91     
==========================================
+ Hits        21147    21212      +65     
- Misses       4334     4360      +26     

Impacted Files	Coverage Δ
panel/config.py	61.29% <ø> (ø)
panel/models/json_editor.py	83.33% <83.33%> (ø)
panel/widgets/__init__.py	100.00% <100.00%> (ø)
panel/widgets/misc.py	95.08% <100.00%> (+0.74%)	⬆️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/viewable.py	69.36% <0.00%> (-1.09%)	⬇️
panel/pane/markup.py	96.17% <0.00%> (-1.05%)	⬇️
panel/tests/pane/test_plotly.py	98.15% <0.00%> (-0.62%)	⬇️
panel/widgets/input.py	96.29% <0.00%> (-0.46%)	⬇️
panel/param.py	86.83% <0.00%> (-0.21%)	⬇️
... and 8 more

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