bundler: allow aliases in `bundle exec`

This means that if you have, for example, `alias rs='rails server'`, you can
run `be rs` and have it expanded to `bundle exec rails server`.

Fixes #5818

plugins/bundler/bundler.plugin.zsh

-alias be="bundle exec"
+alias be="bundle exec "
 alias bl="bundle list"
 alias bp="bundle package"
 alias bo="bundle open"