fix: move wp-prettier from devDependencies to dependencies

[GaryJones]

Summary

• Moves prettier (npm:wp-prettier@3.0.3) from devDependencies to dependencies

Problem

Projects using this plugin need wp-prettier (not regular prettier) to correctly handle WordPress-specific formatting options like parenSpacing: true.

When wp-prettier was only in devDependencies, eslint-plugin-prettier would resolve its peer dependency to regular prettier, causing eslint --fix to incorrectly remove spaces inside parentheses:

// WordPress style (correct)
$( this ).addClass( 'hidden' )

// Gets incorrectly "fixed" to:
$(this).addClass('hidden')

Test plan

• npm test passes
• npm run lint passes
• Verify in a consuming project that npm ls prettier shows wp-prettier under eslint-plugin-prettier

Fixes #235

🤖 Generated with Claude Code

[GaryJones]

Workaround until this is merged

Projects can use npm overrides in their package.json to force the correct prettier version:

{
  "overrides": {
    "@automattic/eslint-plugin-wpvip": {
      "eslint-plugin-prettier": {
        "prettier": "npm:wp-prettier@3.0.3"
      }
    }
  }
}

After adding this, run rm -rf node_modules package-lock.json && npm install to regenerate the lock file.

You can verify it worked with npm ls prettier — you should see wp-prettier@3.0.3 under eslint-plugin-prettier instead of regular prettier@3.x.