'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-position1.zsh

@@ -31,5 +31,5 @@
 BUFFER=$'/'
 
 expected_region_highlight=(
-  '1 1 path' # /
+  '1 1 path_prefix' # /
 )