Add the Excerpt Generation Experiment UI

[dkotter]

What?

Closes #11

This is a follow-up to #96, which added in the Excerpt Generation Experiment and Ability. This adds in the actual admin UI to trigger the generation.

Why?

Coming out of #96 you can manually trigger excerpt generation by hitting the proper API endpoint but there isn't an actual UI to run that. This PR adds in that UI, making it easy for editors to generate excerpts on-demand.

How?

• Loads the proper assets when on the post edit screen
• Uses the __experimentalPluginPostExcerpt slotfill to add a generate button below the excerpt form (note this slotfill is marked as deprecated but as far as I can tell, there isn't another solution right now)
• Uses DOM manipulation to directly inject a generate button into the excerpt panel, as there isn't a slotfill or filter here to use
• Both buttons are tied to the same functionality, which will trigger a request to generate an excerpt and then store that excerpt when returned (or show an error message if an error happens)

Testing Instructions

1. Checkout this PR and run npm i && npm run build
2. Ensure you have valid AI credentials set at Settings > AI Credentials
3. Ensure you turn on the Excerpt Generation Experiment at Settings > AI Experiments
4. Edit a content type that supports excerpts
5. Within the excerpt panel, try generating an excerpt using both buttons and ensure it works

Screenshots or screencast

Test using WordPress Playground

The changes in this pull request can be previewed and tested using this WordPress Playground instance:

Click here to test this pull request.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.27%. Comparing base (6478545) to head (feeb2cb).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
...eriments/Excerpt_Generation/Excerpt_Generation.php	0.00%	17 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #143      +/-   ##
=============================================
- Coverage      46.89%   46.27%   -0.62%     
- Complexity       208      214       +6     
=============================================
  Files             19       19              
  Lines           1271     1288      +17     
=============================================
  Hits             596      596              
- Misses           675      692      +17     

Flag	Coverage Δ
unit	46.27% <0.00%> (-0.62%)	⬇️

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[jeffpaul]

@dkotter perhaps a docs page for this experiment like done in https://github.com/WordPress/ai/pull/155/changes#diff-f70665388390657902e73741dea03efcdadc4bd0ceb4432959f8903ae015c042?