disable z() if fasd is enabled

loading fasd clobbers z(), and results in a cryptic syntax error
message, since 'z' is already an alias when z.plugin.bash is being
sourced.

this conditional detects this situation and returns early.

plugins/available/z.plugin.bash

@@ -15,6 +15,11 @@ about-plugin '                              z is DEPRECATED, use fasd instead'
 #   * z -t foo  # goes to most recently accessed dir matching foo
 #   * z -l foo  # list all dirs matching foo (by frecency)
 
+if [ -h $BASH_IT/plugins/enabled/fasd.plugin.bash ]; then
+    printf '%s\n' 'sorry, the z plugin is incompatible with the fasd plugin. you may use either, but not both.'
+    return
+fi
+
 z() {
  local datafile="$HOME/.z"
  if [ "$1" = "--add" ]; then