Add convert to VegaLiteView

[ahuang11]

A step towards #1461 so the LLMs can see what they plotted and adjust the plot based on the image.

[codecov]

Codecov Report

❌ Patch coverage is 21.51163% with 135 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.70%. Comparing base (77e4af2) to head (a0a2129).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/tests/views/test_base.py	24.32%	112 Missing ⚠️
lumen/views/base.py	4.16%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1462      +/-   ##
==========================================
- Coverage   47.90%   47.70%   -0.20%     
==========================================
  Files         122      122              
  Lines       20799    20974     +175     
==========================================
+ Hits         9963    10005      +42     
- Misses      10836    10969     +133     

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[ahuang11]

Superseded by holoviz/panel#8266