[Feature] Provide v5 dark mode theme to be used with ECharts v6

### What problem does this feature solve?

With version 6.0.0 of ECharts it's possible to use v5 light theme via v5.js.

But how to use v5 dark mode?

Please provide **v5dark.js** to bring back dark theme looking same as in v5.

### What does the proposed API look like?

Provide file v5dark.js to be used in same was as v5.js.