[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.

echarts/src/util/types.ts

Line 828 in 9cbbe57

export type ThemeOption = Dictionary<any>;

Previous issue: #18770, #19159

What does the proposed API look like?

Provide an explicit interface for themes.