feat: Add dynamic-height virtualization

[scdekov]

This PR adds opt-in support for variable-height rows when virtual is enabled. It measures rendered rows, maintains cumulative offsets, and uses binary search to map scrollTop to item indices, ensuring smooth, accurate scrolling even when content wraps or custom renders change row height.

The main motivation is to allow showing longer json string values wrapped so that they're more readable compared to having a long horizontal scroll when using virtual scrolling.

closes: #286

[leezng]

@scdekov Thank you so much! This is a great feature.

However, I've noticed a problem with this commit: component rendering becomes sluggish when the browser viewport width changes, such as by dragging and resizing the browser. Could you help fix this?

The latest version doesn't have this lag; see https://leezng.github.io/vue-json-pretty/

[scdekov]

Hi @leezng thanks for checking the PR! From what I see the sluggishness is coming from the native text area as I'm putting some big chunk of text there. Commenting out the textarea from the example component seems to be resolving the issue.

I guess if we add wrap="off" to the textarea it get's better. If we also reduce the text there it works normally. Any preference on how to go about it?

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