patch 8.2.3263: Vim9: "..=" does not accept same types as the ".." operator

Problem:    Vim9: "..=" does not accept same types as the ".." operator.
Solution:   Convert value to string like ".." does. (issue #8664)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index c6a2965..cafbd6b 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -7086,18 +7086,23 @@
 	    type_T	    *stacktype;
 
 	    if (*op == '.')
-		expected = &t_string;
+	    {
+		if (may_generate_2STRING(-1, FALSE, cctx) == FAIL)
+		    goto theend;
+	    }
 	    else
+	    {
 		expected = lhs.lhs_member_type;
-	    stacktype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-	    if (
+		stacktype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
+		if (
 #ifdef FEAT_FLOAT
-		// If variable is float operation with number is OK.
-		!(expected == &t_float && stacktype == &t_number) &&
+		    // If variable is float operation with number is OK.
+		    !(expected == &t_float && stacktype == &t_number) &&
 #endif
 		    need_type(stacktype, expected, -1, 0, cctx,
 							 FALSE, FALSE) == FAIL)
-		goto theend;
+		    goto theend;
+	    }
 
 	    if (*op == '.')
 	    {