[Feature] Provide an explicit interface for themes

[malobre]

What problem does this feature solve?

Themes have zero documentation, no type definitions (hence no intellisense), this makes it hard to write new theme outside of the theme builder.

https://github.com/apache/echarts/blob/9cbbe57a00aabbe2a5fd30e0d74bf172a318e196/src/util/types.ts#L828

Previous issue: #18770, #19159

What does the proposed API look like?

Provide an explicit interface for themes.