React-tokens to include fallback value for charts

[dlabrecq]

In order for charts to support the dark theme, the chart theme must use CSS variables. In order for the variables to work, devs must also include PatternFly's chart CSS in their pages. This would be a breaking change, unless the variables include a fallback value, like so:

Before

export const chart_color_blue_100: {
  "name": "--pf-chart-color-blue-100",
  "value": "#8bc1f7",
  "var": "var(--pf-chart-color-blue-100)"
};

After

export const chart_color_blue_100: {
  "name": "--pf-chart-color-blue-100",
  "value": "#8bc1f7",
  "var": "var(--pf-chart-color-blue-100, #8bc1f7)"
};

That said, we would like to change React-tokens to include the fallback value for charts.