'main': Allow newlines in command position.

Fixes #501.

Fixes #616 (the original form; not the form in
test-data/alias-comment1.zsh which is now considered o be #677 (see
previous commit for details)).

Fixes a latent bug in test-data/always2.zsh.

No user-visible effect, and therefore, no changelog entry.

highlighters/main/test-data/empty-command-newline.zsh

@@ -34,6 +34,6 @@ BUFFER=$':;\n:'
 expected_region_highlight=(
   '1 1 builtin' # :
   '2 2 commandseparator' # ;
-  '3 3 commandseparator "issue #616"' # \n
+  '3 3 commandseparator' # \n
   '4 4 builtin' # :
 )