feat: add defaults to theme-urls.json output on build

[adamstankiewicz]

Description

Extends build-scss.js Node.js script to include optional --defaultThemeVariants option to customize the default theme variants to include in the theme-urls.json output.

node build-scss.js --defaultThemeVariants light
node build-scss.js --defaultThemeVariants light dark

Ensures the theme-urls.json output looks like the following, with the additional of the defaults object property in favor of the previous default and dark properties:

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

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Merge Checklist

• If your update includes visual changes, have they been reviewed by a designer? Send them a link to the Netlify deploy preview, if applicable.
• Does your change adhere to the documented style conventions?
• Do any prop types have missing descriptions in the Props API tables in the documentation site (check deploy preview)?
• Were your changes tested using all available themes (see theme switcher in the header of the deploy preview, under the "Settings" icon)?
• Were your changes tested in the example app?
• Is there adequate test coverage for your changes?
• Consider whether this change needs to reviewed/QA'ed for accessibility (a11y). If so, please add wittjeff and adamstankiewicz as reviewers on this PR.

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[edx-semantic-release]

🎉 This PR is included in version 21.0.0-alpha.40 🎉

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

Your semantic-release bot 📦🚀