fix: #3588556 Modernize bs-button-variant: delegate to BS5 button-variant() mixin.

Ivica Puljicrequested to merge
issue/bs_base-3588556:3588556-modernize-bs-button-variant-mixin into 2.0.x

fix: #3588556 Modernize bs-button-variant: delegate to BS5 button-variant() mixin.

Wraps BS5's button-variant() with our argument names. Button focus now uses BS5's outer focus ring on :focus-visible instead of the legacy BS3/4 inset depressed look.

By: pivica

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