urdfdom_headers: add livecheck

[samford]

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As mentioned in #158963 (comment), urdfdom uses tags to indicate a new version, despite upstream having created a few GitHub releases in the past. This adds a livecheck block using the same approach (and comment) from #158963, to align urdfdom_headers.

livecheck already checks the Git tags for urdfdom_headers by default and returns 1.1.1 as the newest version but the matches also include a one-off h match (from the pass_through tag). Using the standard regex for Git tags like 1.2.3/v1.2.3 (/^v?(\d+(?:\.\d+)+)$/i) avoids that unwanted tag.

[scpeters]

looks good; thanks!