Add export method to Vega pane

[ahuang11]

Export vega lite to formats

import panel as pn
pn.extension('vega')

vegalite = {
    "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
    "data": {"url": "https://raw.githubusercontent.com/vega/vega/master/docs/data/barley.json"},
    "mark": "bar",
    "encoding": {
        "x": {"aggregate": "sum", "field": "yield", "type": "quantitative"},
        "y": {"field": "variety", "type": "nominal"},
        "color": {"field": "site", "type": "nominal"},
    },
    "width": 800
}
vega = pn.pane.Vega(vegalite, width=800)
pn.pane.PNG(vega.export("png"), width=400, height=200).show()

Todo:

• Update reference guide

[codecov]

Codecov Report

❌ Patch coverage is 84.51327% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.02%. Comparing base (7022916) to head (7c97d82).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/pane/vega.py	61.95%	35 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8266      +/-   ##
==========================================
+ Coverage   85.40%   86.02%   +0.62%     
==========================================
  Files         348      348              
  Lines       54152    54374     +222     
==========================================
+ Hits        46246    46773     +527     
+ Misses       7906     7601     -305     

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[philippjfr]

Let's maybe support as_pane: bool = False and then automatically wrap the returned type in the appropriate pane.

[philippjfr]

@ahuang11 if you could, please parameterize the tests and add docs.