Add example of defining and using a plugin with the embedded component

[cmdcolin]

The current example in storybook fetches the plugin from an external URL

It was difficult to e.g. npm require the plugin, but we can actually define one inline (and this is actually useful for users too, see #2261

This adds an example similar to #2261 for reference

[codecov]

Codecov Report

Merging #2437 (9aff224) into main (6d9b1c2) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 9aff224 differs from pull request most recent head 8c88ecb. Consider uploading reports for the commit 8c88ecb to get more accurate results

@@           Coverage Diff           @@
##             main    #2437   +/-   ##
=======================================
  Coverage   60.83%   60.84%           
=======================================
  Files         538      538           
  Lines       24870    24870           
  Branches     5838     5838           
=======================================
+ Hits        15130    15131    +1     
+ Misses       9424     9423    -1     
  Partials      316      316           

Impacted Files	Coverage Δ
products/jbrowse-web/src/util.ts	27.27% <0.00%> (ø)
packages/core/util/analytics.ts	90.69% <0.00%> (+2.32%)	⬆️

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