Fix issue #15832

[Karlasa]

Description

Added new less variables for primary button to change font-weight, font-size, font-family without changing default button attributes.

Fixed Issues (if relevant)

1. No button-primary__font-weight #15832: No button-primary__font-weight

Manual testing scenarios

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @Karlasa. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.