Add initial E2E setup and tests

[dkotter]

What?

Closes #80

Adds in the initial scaffolding to run E2E tests via Playwright and adds our initial tests

Why?

We currently have no E2E tests and it would be ideal to get those in place so any Experiments we add in the future can have proper test coverage.

How?

• Sets up the initial test configuration using Playwright, following what is done within the Gutenberg repo
• Runs these tests as part of our test GitHub Action workflow
• Adds initial tests for the following:
  • Plugin can be deactivated properly
  • Plugin can be activated properly
  • Both the AI Credentials and AI Experiments settings pages are accessible
  • Can properly add credentials on the AI Credentials setting page
  • Can see proper error messages on the AI Experiments page, both when no credentials have been added and when invalid credentials are added
  • Can globally enable Experiments
  • Ensures Title Generation works as expected
• Adds a request mocking plugin and installs that when the E2E environment is spun up. Right now this plugin will make the OpenAI Models response (other responses to come)

Testing Instructions

Ensure the E2E tests run and pass on this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.89%. Comparing base (7dbbc44) to head (6478545).
⚠️ Report is 31 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #162   +/-   ##
==========================================
  Coverage      46.89%   46.89%           
  Complexity       208      208           
==========================================
  Files             19       19           
  Lines           1271     1271           
==========================================
  Hits             596      596           
  Misses           675      675           

Flag	Coverage Δ
unit	46.89% <ø> (ø)

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[github-actions]

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Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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