Migrate Playground from SASS to PostCSS

[elchininet]

Package typed-scss-modules is without maintenance and it is for more than two years provoking warnings:

Warning: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0

This pull request makes a migration of the Playground package to get rid of typed-scss-modules and also from SASS:

• Removed typed-scss-modules and replaced it by typescript-plugin-css-modules that will check the types during development instead of creating source d.ts types.
• Removed sass and sass-loader and replaced them by postcss plugins (postcss-import, postcss-simple-vars, postcss-nested and postcss-mixins) because the project was already using postcss for auto-prefixing the declarations.
• Removed the generate-types and generate-types:dev scripts because they are not needed after removing typed-scss-modules.
• Make the necessary changes in the code to support the migration.
• Removed the plop config and templates because they were used during development and they are not used anymore.
• Refactor the tsconfig.json file to make it compatible with TypeScript 6.

[coveralls]

coverage: 100.0%. remained the same — playground/migrate_from_sass_to_postcss into master