fix: updates the paragon-theme.json output

[adamstankiewicz]

Description

Updates the paragon-theme.json output during npm run build to look like:

{
  "themeUrls": {
    "core": {
      "paths": {
        "default": "./core.css",
        "minified": "./core.min.css"
      }
    },
    "variants": {
      "light": {
        "paths": {
          "default": "./light.css",
          "minified": "./light.min.css"
        },
        "default": true,
        "dark": false
      }
    }
  }
}

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[edx-semantic-release]

🎉 This PR is included in version 21.0.0-alpha.30 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[edx-semantic-release]

🎉 This PR is included in version 22.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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[openedx-semantic-release-bot]

🎉 This PR is included in version 23.0.0-alpha.1 🎉

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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