'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/abspath-in-command-position3b.zsh

@@ -32,7 +32,7 @@ setopt autocd
 BUFFER=$'/bin; /bin'
 
 expected_region_highlight=(
-  '1 4 path' # /bin (in middle)
+  '1 4 arg0' # /bin (in middle)
   '5 5 commandseparator' # ;
-  '7 10 path' # /bin (at end)
+  '7 10 arg0' # /bin (at end)
 )