refactor: CSS "condition" to SCSS condition

[mfranzke]

Proposed changes

Currently we've checked for the elements to be either input or textarea at the CSS level with :is(). Instead as we're using those in a SCSS mixin, we should that for check for those element types in a SCSS condition.

The main purpose of this is to prevent resulting selectors like .db-input input:is(input,textarea):not(:disabled):read-only, which becomes .db-input input:not(:disabled):read-only

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

[github-actions]

🔭🐙🐈 Test this branch here: https://db-ux-design-system.github.io/core-web/review/refactor-css-condition-to-scss-condition

[Copilot]

Pull Request Overview

This refactoring moves CSS element type checking from runtime CSS :is() selectors to build-time SCSS conditions, improving maintainability and performance by reducing CSS complexity. The change replaces CSS pseudo-selectors with SCSS conditional compilation.

• Replaced CSS :is(input, textarea) selectors with SCSS @if conditions using list.index()
• Added SCSS variables to define valid form field types for read-only and field-sizing properties
• Updated CSS variable naming to be dynamic based on the element type