CSS: Add validation icon CSS var

[louismaximepiton]

Description

Adding a CSS variable for validation icons in the form.
Introduce the new variable in the validation process.
Change the CSS variable in dark mode.
⚠️ Change forms CSS variables to import them only when needed

Motivation & Context

Better consistency for dark mode on validating forms

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39107--twbs-bootstrap.netlify.app/docs/5.3/forms/validation

Related issues

Might close some of #37549.