Fix for numbered list item when exporting to PDF, DOCX

[mfanselmo]

This PR #1326 introduced the ability to start numbered lists from indices other than 1. I noticed on the demo to export to pdf/docx this was not taked into account.

I was not able to run playwright locally and update snapshots if that is needed, can you give me guidance here or do it?

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[YousefED]

Nice, thanks. We'll fix the playwright tests, I think it's unrelated. Could you:

• also add support to the docx exporter?
• add it to the unit tests of the exporters? (add a case to the default test case and update the related snapshots)

[mfanselmo]

Hey! I spent quite a bit of time on this but ran into some tricky edge cases I couldn’t get to work perfectly. Let me break them down:

For PDFs, numbering is straightforward since we can manually set the numbering. However, Word has a few quirks:
To start a numbered list at a value other than 1, you have to define a specific configuration for that list type. It’s not as simple as just setting a start value. I handled this by maintaining a set of starting indices and manually referencing them via blocknote-numbered-list-{index}.
Word automatically increments list numbers, so if you need to restart numbering manually, you have to use the instance parameter for adjacent lists.
I came across a weird edge case where adjacent lists, nested under different parent blocks, don’t behave as expected—even though the instance parameter should fix it. For now, my code assigns a unique numeric ID to each list instance, but I believe this might be overkill and could be simplified. I left it in just in case someone wants to dig into it.

Here’s an example of content that doesn’t render correctly in Word. Based on how instance works, I would expect this to work fine:

I’ve updated the snapshots for both PDF and DOCX to test these cases. The PDF output works perfectly, but DOCX still has this one issue. @YousefED I will leave this one to this point for now, but if I find time in the future I will give it another go

[mfanselmo]

This code is a bit messy and could definitely be improved, but its the best I could come up with to ensure a unique instance id is given to each numbered list. In theory this id should guarantee no weird continuations of non adjacent lists happen, but for some reason they still are. I would not merge this as is