[Bug] Kirki Font Control for Headings uses incorrect property "variant" instead of "font-variant"

[jessecowens]

Describe the bug
Fonts that use a different weight than "400" for their bold variant do not adjust when their variant is chosen in the customizer. This is the style that is produced:

<style id="kirki-postmessage-bgtfw_headings_typography">h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6, .bgc-heading.bgc-page-title{font-family:Old Standard TT;}h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6, .bgc-heading.bgc-page-title{line-height:1.1;}h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6, .bgc-heading.bgc-page-title{text-transform:uppercase;}h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6, .bgc-heading.bgc-page-title{variant:700;}h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6, .bgc-heading.bgc-page-title{font-style:normal;}</style>

The variant:700 is not a valid CSS property, it should be font-variant:700

To Reproduce
Steps to reproduce the behavior:

1. Go to Customize > Fonts.
2. For the Headings, choose a Google Font that has a different variant weight, for example Old Standart TT
3. Select 700
4. Note that heading font weight does not change

Expected behavior
The inline CSS from Kirki should use font-variant to correctly adjust the weight of the font.

Screenshots

[nicolepaschen]

Duplicate of #307