Export wikilinkUrl for wikilink resolution

[srid]

Export wikilinkUrl function from Commonmark.Extensions.WikiLink module.
This allows consumers to extract the URL from a WikiLink value, which is useful for building resolution maps.

Also added a test case in test/Commonmark/Extensions/WikiLinkSpec.hs to verify the export and functionality.

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PR created automatically by Jules for task 7689097402921709998 started by @srid

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