'main': In command position, do not highlight directories (unless AUTO_CD is set) and non-executable files.

Fixes #202.

Test expectations are updated.  For example, BUFFER='/bin' is now
highlighted as path_prefix because it's a prefix of '/bin/sh' which
would be valid.  However, BUFFER='/bin;' is now properly highlighted
as an error (unless AUTO_CD is set).

highlighters/main/test-data/path-dollared-word3.zsh

@@ -34,7 +34,7 @@
 BUFFER='$PWD; ${PWD}'
 
 expected_region_highlight=(
-  "1 4 path" # $PWD
+  "1 4 unknown-token" # $PWD (without AUTO_CD)
   "5 5 commandseparator" # ;
-  "7 12 path" # ${PWD}
+  "7 12 path_prefix" # ${PWD}
 )