Wildcard [Charts]: Move hardcoded active bar styles to the public API

Created by: vovakulikov

Follow-up for https://github.com/sourcegraph/sourcegraph/pull/40138

In https://github.com/sourcegraph/sourcegraph/pull/40138 we introduced a different visual mode for the active bar (hovered/focused). We made active bar styles hardcoded right in the built-in <BarChart /> component (currently it changes opacity to 0.5 value). In order to increase flexibility, we should move ou active bar styles to the public API.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin