feat(tailwind): add transparent to supported colors

What does this MR do?

transparent must be explicitly defined as a supported color in Tailwind's config to enable utils such as .gl-bg-transparent and the likes.

How to test this

1. You can run the Tailwind Config Viewer right here: yarn tailwind-config-viewer.
2. To see how this affects the CSS in Js in GitLab:
   1. Checkout the master branch and ensure everything is up-to-date: yarn && rm config/helpers/tailwind/css_in_js.js && yarn tailwindcss:build.
   2. Stage the CSS in Js config: git add --force config/helpers/tailwind/css_in_js.js.
   3. Install this MR's build: yarn add @gitlab/ui@https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/6611874550/artifacts/raw/gitlab-ui.tailwind-preset-color-transparent.tgz
   4. Rebuild the CSS in Js: rm config/helpers/tailwind/css_in_js.js && yarn tailwindcss:build.
   5. You should see the following diff:

   diff --git a/config/helpers/tailwind/css_in_js.js b/config/helpers/tailwind/css_in_js.js
   index 17e96fd3f118..07bddf8ce936 100644
   --- a/config/helpers/tailwind/css_in_js.js
   +++ b/config/helpers/tailwind/css_in_js.js
   @@ -1,5 +1,5 @@
   /**
   - * The following 675 definitions need to be migrated to Tailwind.
   + * The following 670 definitions need to be migrated to Tailwind.
   * Let's do this! 🚀
   */
   module.exports = {
   @@ -32,7 +32,6 @@ module.exports = {
         'animation-iteration-count': '3',
      },
      },
   -  '.bg-transparent': { 'background-color': 'transparent' },
      '.hover-bg-transparent': { '&:hover': { 'background-color': 'transparent' } },
      '.bg-transparent\\!': { 'background-color': 'transparent !important' },
      '.hover-bg-transparent\\!': { '&:hover': { 'background-color': 'transparent !important' } },
   @@ -105,8 +104,6 @@ module.exports = {
      '.border-b-solid\\!': { 'border-bottom-style': 'solid !important' },
      '.border-b-initial': { 'border-bottom-style': 'initial' },
      '.border-l-solid': { 'border-left-style': 'solid' },
   -  '.border-transparent': { 'border-color': 'transparent' },
   -  '.border-t-transparent': { 'border-top-color': 'transparent' },
      '.border-t-transparent\\!': { 'border-top-color': 'transparent !important' },
      '.hover-border-gray-100': { '&:hover': { 'border-color': 'var(--gray-100, #dcdcde)' } },
      '.border-gray-100\\!': { 'border-color': 'var(--gray-100, #dcdcde) !important' },
   @@ -215,7 +212,6 @@ module.exports = {
      },
      '.reset-color': { color: 'inherit' },
      '.reset-color\\!': { color: 'inherit !important' },
   -  '.text-transparent': { color: 'transparent' },
      '.text-white\\!': { color: 'var(--white, #fff) !important' },
      '.text-body': { color: 'var(--gl-text-primary, #333238)' },
      '.text-body\\!': { color: 'var(--gl-text-primary, #333238) !important' },
   @@ -678,7 +674,6 @@ module.exports = {
      '.lg-pt-3': { '@media (min-width: 992px)': { 'padding-top': '0.5rem' } },
      '.lg-pr-5': { '@media (min-width: 992px)': { 'padding-right': '1rem' } },
      '.fill-current-color': { fill: 'currentColor' },
   -  '.fill-transparent': { fill: 'transparent' },
      '.text-left\\!': { 'text-align': 'left !important' },
      '.text-center\\!': { 'text-align': 'center !important' },
      '.reset-text-align': { 'text-align': 'inherit' },

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Accessibility

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• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
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