Support different line heights for buttons.

[LunicLynx]

Description

Buttons of different sizes should also support different line heights.

Motivation & Context

When configuring a custom style it is required to also adjust the line height for different size buttons.

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-38062--twbs-bootstrap.netlify.app/

[LunicLynx]

Maybe this should also be done for control sizes as well.

[julien-deramond]

Thanks for this PR @LunicLynx. We'll try to look into it asap.
Just for info for the possible reviewers, Boosted (fork of Bootstrap) has the same approach to handle the line height: new parameter in button-size() mixin + $btn-line-height-lg and $btn-line-height-sm.