You can find those operators here: julia/julia-parser.scm at master · JuliaLang/julia · GitHub
Because superscripts are already allowed for variable names, making them act as exponents (or ᵀ for transpose) would be a breaking change and would violate semantic versioning if done prior to v2.0. At its current stage of development, Julia is unlikely to add new syntax sugar except in cases where it does not cause a breaking change. As demonstrated by Python 2 → Python 3 migration hell, language developers need to be very conservative with breaking changes to limit the cost they impose on users.