fix: support multiple overridden syntax themes

[jay3332]

Fixes #1974

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[dimaMachina]

could you add an example with 2 themes in examples/docs?

[dimaMachina]

I decided to not add support for multiple themes, since rendering multiple code blocks for each theme is awful.

However, in the future, multiple themes will be supported. You can track the progress in shikiji (newly fork of Shiki) https://github.com/antfu/shikiji#multiple-themes that already supports multiple themes without rendering multiple code blocks.

Also, I will merge your contribution to syntax docs with Custom Grammar and Custom Themes sections