Create block: Add CSS templates

[Infinite-Null]

What?

Adds CSS variant support to the @wordpress/create-block package, allowing developers to scaffold blocks with CSS files instead of SCSS files.
Closes: #52181

Why?

The current create-block package defaults to SCSS files, but some developers prefer working with plain CSS files for consistency and standards enforcement. While CSS can be written in SCSS files, having dedicated CSS templates provides a better developer experience and enforces coding standards from the start.

Testing Instructions

1. Test CSS static variant:

 npx @wordpress/create-block my-css-block --variant css

2. Verify it generates CSS files (editor.css, style.css) instead of SCSS
3. Verify that JavaScript files import CSS files correctly

Screencast:

Screen.Recording.2025-07-03.at.6.34.59.PM.mov

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Co-authored-by: Infinite-Null <ankitkumarshah@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: benoitchantre <benoitchantre@git.wordpress.org>

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