patch 8.2.2784: Vim9: cannot use \=expr in :substitute

Problem:    Vim9: cannot use \=expr in :substitute.
Solution:   Compile the expression into instructions and execute them when
            invoked.

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2784,
 /**/
     2783,
 /**/