Introduce 'tag-link-position-x' class to tag cloud links.

The new class describes the cardinal position of a link in the cloud, allowing
more fine-grained CSS and JS targeting.

Props Mte90, chmac.
Fixes #5172.
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git-svn-id: https://core.svn.wordpress.org/trunk@35949 1a063a9b-81f0-0310-95a4-ce76da25c4cd