feat(ui): "append" parameter for "msg_show" UI events

Problem:  Consecutive "msg_show" events stemming from an `:echon`
          command are supposed to be appended without a newline, this
          information is not encoded in the "msg_show" event.
Solution: Add an "append" parameter to the "msg_show" event that is set
          to true to indicate the message should not start on a new line.
Considered alternative: Emit a newline for the common case instead at the
start of a new message. That way UIs can more closely follow the logic
as it is implemented for the message grid currently. This would be a
breaking change. The "append" parameter seems OK.

src/nvim/api/ui_events.in.h

@@ -160,7 +160,7 @@ void wildmenu_select(Integer selected)
 void wildmenu_hide(void)
   FUNC_API_SINCE(3) FUNC_API_REMOTE_ONLY;
 
-void msg_show(String kind, Array content, Boolean replace_last, Boolean history)
+void msg_show(String kind, Array content, Boolean replace_last, Boolean history, Boolean append)
   FUNC_API_SINCE(6) FUNC_API_FAST FUNC_API_REMOTE_ONLY;
 void msg_clear(void)
   FUNC_API_SINCE(6) FUNC_API_REMOTE_ONLY;

src/nvim/eval.c

@@ -7863,6 +7863,7 @@ void ex_echo(exarg_T *eap)
       char *tofree = encode_tv2echo(&rettv, NULL);
       if (*tofree != NUL) {
         msg_ext_set_kind("echo");
+        msg_ext_append = eap->cmdidx == CMD_echon;
         msg_multiline(cstr_as_string(tofree), echo_hl_id, true, false, &need_clear);
       }
       xfree(tofree);

src/nvim/message.c

@@ -3185,7 +3185,8 @@ void msg_ext_ui_flush(void)
   msg_ext_emit_chunk();
   if (msg_ext_chunks->size > 0) {
     Array *tofree = msg_ext_init_chunks();
-    ui_call_msg_show(cstr_as_string(msg_ext_kind), *tofree, msg_ext_overwrite, msg_ext_history);
+    ui_call_msg_show(cstr_as_string(msg_ext_kind), *tofree, msg_ext_overwrite, msg_ext_history,
+                     msg_ext_append);
     if (msg_ext_history || strequal(msg_ext_kind, "return_prompt")) {
       api_free_array(*tofree);
     } else {
@@ -3205,6 +3206,7 @@ void msg_ext_ui_flush(void)
     }
     msg_ext_overwrite = false;
     msg_ext_history = false;
+    msg_ext_append = false;
     msg_ext_kind = NULL;
   }
 }

src/nvim/message.h

@@ -33,6 +33,8 @@ extern MessageHistoryEntry *msg_hist_last;
 EXTERN bool msg_ext_need_clear INIT( = false);
 // Set to true to force grouping a set of message chunks into a single `cmdline_show` event.
 EXTERN bool msg_ext_skip_flush INIT( = false);
+// Set to true when message should be appended to previous message line.
+EXTERN bool msg_ext_append INIT( = false);
 
 /// allocated grid for messages. Used unless ext_messages is active.
 /// See also the description at msg_scroll_flush()