In vim just do (copy and paste it if you don't know how to get the initial ⁏):

   :imap <buffer> l ⁏

and whenever you want an inverted semi, type l. For best effects put that line in your ftplugin directory, in semi.vim (or whatever your autodetect filetype name for semicolon is).

some people like to map j to ⁏ and k to ;, other like to map f to ⁏ and j to ;, but this is all just homerow keeping. I'm a purist, i like to keep the mappings minimal.