New Sass var $btn-link-focus-shadow-rgb to allow customisation

[giovannipiller]

Description

Adds $btn-link-focus-shadow-rgb so that users can customise focus shadow for .btn-link.

Motivation & Context

Right now, for .btn-link, it's generated using only $link-color.

--#{$prefix}btn-focus-shadow-rgb: #{to-rgb(mix(color-contrast($link-color), $link-color, 15%))};

This might be undesirable and there should be an easy way to override it (e.g. users might want to change how it's calculated, or have specific design requirements that differ between light/dark theme).

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-39119--twbs-bootstrap.netlify.app/docs/5.3/components/buttons/

[louismaximepiton]

Something has to be rethought in order not to increase the bundles size too much.

[giovannipiller]

Something has to be rethought in order not to increase the bundles size too much.

That's fair. I'm going to simply introduce a new Sass variable then, as you suggested: $btn-link-focus-shadow-rgb.
This should give enough flexibility to users that want to customise the shadow.

[louismaximepiton]

Thanks for this PR!