Promote Image Generation from an Experiment to a Feature

[dkotter]

What?

Partially closes #234, #416

At a code-level, change the Image Generation Feature to be a stable Feature instead of an Experimental Feature.

Why?

The groundwork was laid in #316 to distinguish between an experimental Feature and a stable Feature. We left all Features as experimental but now want to promote certain ones that we feel have been thoroughly tested and bring significant value. We are starting with the Image Generation (which also includes Image Editing) Feature.

Note this only modifies things at the code-level, not the UI. Those changes are coming in #417. Worth noting that until that PR is merged, the Image Generation Feature will start to show in an existing "Other Features" section (from the current Editor Experiments section):

How?

• Move the Image_Generation class from the Experiments directory to the Features directory
• Mark this Feature as stable and remove the Experiment category of Editor
• Add a new features directory within our src directory and move all Image Generation JS code here
• Ensure this Feature is loaded properly and removed from loading as an Experiment
• Move all tests (E2E and unit) into a new Features directory and out of the Experiments directory for consistency
• Update docs

Use of AI Tools

None

Testing Instructions

1. Pull this PR down and run npm i && npm run build
2. Go to the settings page and ensure you no longer see Image Generation and Editing in the Editor Experiments section but you do see it in the Other Features section (again, this is temporary until Add visual-card presentation for featured experiments #417 is done)
3. Turn this Feature on and test it thoroughly, ensuring you have a Provider connected (like Google) that supports image generation and editing

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[dkotter]

Note I removed this category as we currently have that marked as an Experiment Category, and the only place we use this is determining what section a Feature shows within our settings page (this defaults to "Other" if not set).

In #417 we'll start showing stable Features in a new section up top so I don't think categories are needed anymore for stable Features. That said, we can introduce Feature Categories (maybe matching what we have for Experiment Categories, Editor and Admin) if we think that would still be useful to have

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.75%. Comparing base (cf338eb) to head (22ffbe1).

Files with missing lines	Patch %	Lines
...des/Features/Image_Generation/Image_Generation.php	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #418   +/-   ##
==========================================
  Coverage      65.75%   65.75%           
  Complexity       763      763           
==========================================
  Files             53       53           
  Lines           3863     3863           
==========================================
  Hits            2540     2540           
  Misses          1323     1323           

Flag	Coverage Δ
unit	65.75% <50.00%> (ø)

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