Add SASS variable $display-font-family

Created by: SamuelMiller

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

Currently, setting a custom $headings-font-family in scss only affects h1-h6 tags and their classes, not the .display class, which can be used as a type of heading.

Please consider either including the display class when building heading font families with $headings-font-family, or perhaps better, provide a "$display-font-family" variable to allowing customizing the display class font-family specifically. Thanks.

Motivation and context

To allow the user to customize all the default Bootstrap headings.