Improve use of CSS vars in Toasts

Description

• Create a new --bs-toast-spacing CSS var linked to $toast-spacing
• Use --bs-toast-font-size in @include font-size()

Motivation & Context

Get rid of all remaining $toast-* SCSS vars in _toasts.scss.

Types of changes

• Refactoring (non-breaking change)

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
• All new and existing tests passed

Related issues

N/A

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