This feature made it into the latest beta release, so please upgrade and give it a try.
Creating synonyms and merging tags is done in the same place. If you wanted to merge the “js” and “javascript” tags, and choose “javascript” as the base tag, then go to the javascript tag’s page (/tags/javascript).
Once “js” is added as a synonym, its topics will show as tagged with “javascript”.
In search and filters, js will show up and direct people to javascript instead. Tagging topics will js will use javascript instead. And so on. I hope I caught all the places where synonyms need to be handled, so let me know if I missed anything.