[zsh] move vte magic to `.zshrc`

Ok... as the way I am loading and starting the system, when zprofile is
being sourced, I am from a TTY (see the end of `.zprofile`), and therefore
`$VTE_VERSION` doesn't exist which makes totally sense. Since this only
make sense when using a compatible vte terminal emulator (like everyone
except xterm and rxvt), this should be tested everytime we open a new
interactive shell (hence in `.zshrc`).