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/api/keysets_defs.h

@@ -325,6 +325,7 @@ typedef struct {
 } Dict(cmd_opts);
 
 typedef struct {
+  Boolean err;
   Boolean verbose;
 } Dict(echo_opts);
 

src/nvim/api/private/helpers.c

@@ -776,7 +776,7 @@ char *api_typename(ObjectType t)
   UNREACHABLE;
 }
 
-HlMessage parse_hl_msg(Array chunks, Error *err)
+HlMessage parse_hl_msg(Array chunks, bool is_err, Error *err)
 {
   HlMessage hl_msg = KV_INITIAL_VALUE;
   for (size_t i = 0; i < chunks.size; i++) {
@@ -791,7 +791,7 @@ HlMessage parse_hl_msg(Array chunks, Error *err)
 
     String str = copy_string(chunk.items[0].data.string, NULL);
 
-    int hl_id = 0;
+    int hl_id = is_err ? HLF_E : 0;
     if (chunk.size == 2) {
       hl_id = object_to_hl_id(chunk.items[1], "text highlight", err);
     }

src/nvim/api/vim.c

@@ -775,13 +775,15 @@ void nvim_set_vvar(String name, Object value, Error *err)
 ///                `hl_group` element can be omitted for no highlight.
 /// @param history  if true, add to |message-history|.
 /// @param opts  Optional parameters.
+///          - err: Treat the message like |:echoerr|. Omitted `hlgroup`
+///            uses |hl-ErrorMsg| instead.
 ///          - verbose: Message is printed as a result of 'verbose' option.
 ///            If Nvim was invoked with -V3log_file, the message will be
 ///            redirected to the log_file and suppressed from direct output.
 void nvim_echo(Array chunks, Boolean history, Dict(echo_opts) *opts, Error *err)
   FUNC_API_SINCE(7)
 {
-  HlMessage hl_msg = parse_hl_msg(chunks, err);
+  HlMessage hl_msg = parse_hl_msg(chunks, opts->err, err);
   if (ERROR_SET(err)) {
     goto error;
   }
@@ -790,7 +792,7 @@ void nvim_echo(Array chunks, Boolean history, Dict(echo_opts) *opts, Error *err)
     verbose_enter();
   }
 
-  msg_multihl(hl_msg, history ? "echomsg" : "echo", history);
+  msg_multihl(hl_msg, opts->err ? "echoerr" : history ? "echomsg" : "echo", history, opts->err);
 
   if (opts->verbose) {
     verbose_leave();