feat(api): add err field to nvim_echo() opts

Problem:  We want to deprecate `nvim_err_write(ln)()` but there is no
          obvious replacement (from Lua). Meanwhile we already have
          `nvim_echo()` with an `opts` argument.
Solution: Add `err` argument to `nvim_echo()` that directly maps to
          `:echoerr`.

src/nvim/ex_docmd.c

@@ -982,7 +982,7 @@ void handle_did_throw(void)
   if (messages != NULL) {
     do {
       msglist_T *next = messages->next;
-      emsg_multiline(messages->msg, messages->multiline);
+      emsg_multiline(messages->msg, "emsg", HLF_E, messages->multiline);
       xfree(messages->msg);
       xfree(messages->sfile);
       xfree(messages);