Update react-vega to v8 and adapt VegaChart accordingly

[clarasb]

This PR:

• updates react-vega to v8 and adapts VegaChart accordingly (see rect-vega migration guide)
• removes altair pinning. This is not necessary anymore
• integrates VegaChart component in testing (after beeing excluded in Update vega #131)

Closes #132.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.91%. Comparing base (e295ef8) to head (ad8def6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   92.28%   92.91%   +0.63%     
==========================================
  Files          89       90       +1     
  Lines        1529     1553      +24     
  Branches      235      236       +1     
==========================================
+ Hits         1411     1443      +32     
+ Misses         99       92       -7     
+ Partials       19       18       -1     

Flag	Coverage Δ
backend	100.00% <ø> (ø)
frontend	85.52% <100.00%> (+1.55%)	⬆️

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[b-yogesh]

Thanks Clara, it LGTM! Did you also test this out in a fresh node environment and the demos work as expected?