startup: do "syntax enable" instead of "syntax on"

"syntax on" overwrites existing highlight groups, while "syntax enable"
just sets missing groups. This change prevents user defined highlights in
init.vim/lua to get overwritten. The manual recommends "syntax enable" for
new configurations anyway, "on" command was probably used as it is the
implicit default.

src/nvim/main.c

@@ -375,7 +375,7 @@ int main(int argc, char **argv)
     // Does ":filetype plugin indent on".
     filetype_maybe_enable();
     // Sources syntax/syntax.vim, which calls `:filetype on`.
-    syn_maybe_on();
+    syn_maybe_enable();
   }
 
   // Read all the plugin files.