feat(heatmap): dark mode with theme controls

[nickofthyme]

Summary

Adds dark mode to goal charts with configurable theme styles.

BREAKING CHANGES

The HeatmapSpec.config prop is removed. All properties have been moved/renamed under new Theme.heatmap options with the following exceptions:

• HeatmapSpec.margin was not used, Theme.chartMargins should be used in the future.
• HeatmapSpec.fontFamily HeatmapSpec.width and HeatmapSpec.height all removed as they were never used.

Details

The Heatmap, Partition and WordCloud specs all have no connection to the chart theme and thus have no canonical inheritance to dark mode. This PR fixes this for the Heatmap chart and adds some added styles to the text labels as exposed in xy charts. The HeatmapConfig was broken down and moved into Theme or the HeatmapSpec accordingly. We should avoid having a Config option to control theme as this not only creates a disconnect but also allows for multiple sources of truth for similar options and another property requirement to toggle for theming between dark and light modes.

In the future I think it would great if we have a Config alternative to the react props in addition to the Theme options.

Issues

closes #1335

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• The :theme label has been added and the @elastic/eui-design team has been pinged when there are Theme API changes
• All related issues have been linked (i.e. closes #123, fixes #123)
• New public API exports have been added to packages/charts/src/index.ts
• Unit tests have been added or updated to match the most common scenarios
• The proper documentation and/or storybook story has been added or updated
• The code has been checked for cross-browser compatibility (Chrome, Firefox, Safari, Edge)
• Visual changes have been tested with all available themes including dark, light, eui-dark & eui-light

[markov00]

I can see a visible difference on the echHighlighterMask in this PR in dark mode:
the Y axis and the selected areas are not transparent anymore as in the old implementation:
This PR:

Current master:

Despite the black text, the behaviour on master is the correct one. clearly visible also in the light theme

I'd also like to chat about when a style needs to be pushed into the theme and when it needs to be local on the spec.

[markov00]

just remember that the heatmap is in @alpha and we can also avoid having BREAKING CHANGES there

[nickofthyme]

I'd also like to chat about when a style needs to be pushed into the theme and when it needs to be local on the spec.

Aside from what we discussed in todays meeting, I tend to through all function thing like formatters in the spec rather than the theme.

[markov00]

Looks good to me
tested locally and all the issues are now gone!