'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/plain-file-in-command-position.zsh

@@ -33,7 +33,7 @@ chmod -x foo
 BUFFER=$'./foo; ./foo'
 
 expected_region_highlight=(
-  '1 5 unknown-token "issue #202"' # ./foo (in middle)
+  '1 5 unknown-token' # ./foo (in middle)
   '6 6 commandseparator' # ;
-  '8 12 unknown-token "issue #202"' # ./foo (at end)
+  '8 12 unknown-token' # ./foo (at end)
 )