[Visualization UI] Define fonts and usage guidelines (family, size, weight)

[joshuarrrr]

As part of providing a cohesive visualization experience, and to provide updated look and feel, we need design system guidance on typography within visualizations. While there may be many other typographic visualization considerations (see opensearch-project/OpenSearch-Dashboards#3368 for a partial list), we'll start with the basics listed here as part of opensearch-project/OpenSearch-Dashboards#4239

The deliverables here can be in the form of updated OUI variables or simply documented guidance for typographic usage within visualizations.