Feature request: Expose line number index for color highlighting #600

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Feature request

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What problem does this feature solve?

It makes it possible to start the lines numbers to something else than 1.

What does the proposed API look like?

Exposed in a similar way to the current line highlighting in the markdown.

How should this be implemented in your opinion?

https://github.com/saberland/saber/blob/ec2cbac5a093edbe7454f14a0cfd1af6276c1ab3/packages/saber/src/markdown/highlight-plugin.js#L66

Are you willing to work on this yourself?

I could if no one else does.