patch 8.2.4758: when using an LSP channel want to get the message ID

Problem:    When using an LSP channel want to get the message ID.
Solution:   Have ch_sendexpr() return the ID. (Yegappan Lakshmanan,
            closes #10202)
diff --git a/src/channel.c b/src/channel.c
index 85089cb..ef747d0 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4520,6 +4520,7 @@
     ch_part_T	part_read;
     jobopt_T    opt;
     int		timeout;
+    int		callback_present = FALSE;
 
     // return an empty string by default
     rettv->v_type = VAR_STRING;
@@ -4546,7 +4547,9 @@
     {
 	dict_T		*d;
 	dictitem_T	*di;
-	int		callback_present = FALSE;
+
+	// return an empty dict by default
+	rettv_dict_alloc(rettv);
 
 	if (argvars[1].v_type != VAR_DICT)
 	{
@@ -4629,6 +4632,14 @@
 	}
     }
     free_job_options(&opt);
+    if (ch_mode == MODE_LSP && !eval && callback_present)
+    {
+	// if ch_sendexpr() is used to send a LSP message and a callback
+	// function is specified, then return the generated identifier for the
+	// message.  The user can use this to cancel the request (if needed).
+	if (rettv->vval.v_dict != NULL)
+	    dict_add_number(rettv->vval.v_dict, "id", id);
+    }
 }
 
 /*