Add ECharts pane [doc-build]

[philippjfr]

Adds an ECharts pane built on the echarts.js library. This was very straightforward to do and ECharts looks very polished with nice animated transitions and a number of BI indicator types which will round out the functionality of Panel for building BI type dashboards.

gauge = {
    'tooltip': {
        'formatter': '{a} <br/>{b} : {c}%'
    },
    'series': [
        {
            'name': 'Gauge',
            'type': 'gauge',
            'detail': {'formatter': '{value}%'},
            'data': [{'value': 50, 'name': 'Value'}]
        }
    ]
};
gauge_pane = pn.pane.ECharts(gauge, width=400, height=400)
gauge_pane

Cc: @MarcSkovMadsen

[codecov]

Codecov Report

Merging #1484 into master will increase coverage by 0.13%.
The diff coverage is 78.43%.

@@            Coverage Diff             @@
##           master    #1484      +/-   ##
==========================================
+ Coverage   86.04%   86.18%   +0.13%     
==========================================
  Files         139      141       +2     
  Lines       15567    15706     +139     
==========================================
+ Hits        13395    13536     +141     
+ Misses       2172     2170       -2     

Impacted Files	Coverage Δ
panel/config.py	45.64% <ø> (ø)
panel/pane/echarts.py	73.17% <73.17%> (ø)
panel/models/echarts.py	100.00% <100.00%> (ø)
panel/pane/__init__.py	100.00% <100.00%> (ø)
panel/tests/widgets/test_tables.py	98.11% <0.00%> (+0.79%)	⬆️
panel/widgets/tables.py	95.17% <0.00%> (+8.00%)	⬆️

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