feat: add support for :host pseudo-class in style-dictionary output

[xitij2000]

Description

This PR updates the generated CSS variables from design tokens to emit variables under both :root and :host.

Previously, design token CSS variables were generated only under :root, which makes them globally available at the document level. This works well for normal application usage, but it does not fully support stylesheets that are loaded inside a Shadow DOM.

The primary use case for this change is to support isolated XBlocks that render inside a Shadow DOM. By including :host alongside :root, the XBlock can load these stylesheets with the theme remaining isolated to the XBlock without spilling over to the rest of the page.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.40%. Comparing base (9340b9c) to head (b7cf022).

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.x    #4282   +/-   ##
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  Coverage         94.40%   94.40%           
=============================================
  Files               242      242           
  Lines              4309     4309           
  Branches            981      981           
=============================================
  Hits               4068     4068           
  Misses              237      237           
  Partials              4        4           

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