widget binding: don't exclude widgets starting with '_'

Just because a widget starts with _ does not mean it should be
skipped when rebinding. The only reason widgets need to be skipped
is when their function name start with _zsh_highlight_widget*,
which is checked later.

Example:
_expand_alias (^Xa) needs to be wrapped.

The exclusion of _* was introduced in 186d80054a
to fix #65, but it is not needed. The relevant exclusion of
_zsh_highlight_widget* is already enough due to the case statement.

zsh-syntax-highlighting.zsh

@@ -200,7 +200,7 @@ _zsh_highlight_bind_widgets()
 
   # Override ZLE widgets to make them invoke _zsh_highlight.
   local cur_widget
-  for cur_widget in ${${(f)"$(builtin zle -la)"}:#(.*|_*|orig-*|run-help|which-command|beep|set-local-history|yank)}; do
+  for cur_widget in ${${(f)"$(builtin zle -la)"}:#(.*|orig-*|run-help|which-command|beep|set-local-history|yank)}; do
     case $widgets[$cur_widget] in
 
       # Already rebound event: do nothing.