'main': Expand aliases first. (Issue #264.)

This commit causes an alias to an invalid command to be highlighted as an error
(unknown-token).

highlighters/main/test-data/noglob-alias.zsh

@@ -31,6 +31,6 @@ alias x=command
 BUFFER='x ls'
 
 expected_region_highlight=(
-  "1 1 alias" # x
+  "1 1 precommand" # x
   "3 4 command" # ls
 )