Allow font-variant to be customized

[neemski]

First of all, thank you for your contribution! :-)

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

• Make sure that you propose PR to right branch: bugfix for master, feature for branch feature.
• Make sure that you follow antd's code convention.
• Run npm run lint and fix those errors before submitting in order to keep consistent code style.
• Rebase before creating a PR to keep commit history clear.
• Add some descriptions and refer relative issues for you PR.

Extra checklist:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update TypeScript definition for the component.
• Add unit tests for the feature.

[codecov]

Codecov Report

Merging #12691 into feature will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           feature   #12691   +/-   ##
========================================
  Coverage    92.57%   92.57%           
========================================
  Files          213      213           
  Lines         5601     5601           
  Branches      1605     1605           
========================================
  Hits          5185     5185           
  Misses         410      410           
  Partials         6        6

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