patch 8.2.3913: help for expressions does not mention Vim9 syntax

Problem:    Help for expressions does not mention Vim9 syntax.
Solution:   Add the rules for Vim9 to the expression help.  Rename functions
            to match the help.
diff --git a/src/version.c b/src/version.c
index 06a0a33..658d9be 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3913,
+/**/
     3912,
 /**/
     3911,
diff --git a/src/vim9expr.c b/src/vim9expr.c
index 6a1faf8..1e87b8e 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -1883,7 +1883,7 @@
  *  trailing ->name()	method call
  */
     static int
-compile_expr7(
+compile_expr8(
 	char_u **arg,
 	cctx_T *cctx,
 	ppconst_T *ppconst)
@@ -2119,10 +2119,10 @@
 }
 
 /*
- * <type>expr7: runtime type check / conversion
+ * <type>expr8: runtime type check / conversion
  */
     static int
-compile_expr7t(char_u **arg, cctx_T *cctx, ppconst_T *ppconst)
+compile_expr7(char_u **arg, cctx_T *cctx, ppconst_T *ppconst)
 {
     type_T *want_type = NULL;
 
@@ -2147,7 +2147,7 @@
 	    return FAIL;
     }
 
-    if (compile_expr7(arg, cctx, ppconst) == FAIL)
+    if (compile_expr8(arg, cctx, ppconst) == FAIL)
 	return FAIL;
 
     if (want_type != NULL)
@@ -2182,7 +2182,7 @@
     int		ppconst_used = ppconst->pp_used;
 
     // get the first expression
-    if (compile_expr7t(arg, cctx, ppconst) == FAIL)
+    if (compile_expr7(arg, cctx, ppconst) == FAIL)
 	return FAIL;
 
     /*
@@ -2208,7 +2208,7 @@
 	    return FAIL;
 
 	// get the second expression
-	if (compile_expr7t(arg, cctx, ppconst) == FAIL)
+	if (compile_expr7(arg, cctx, ppconst) == FAIL)
 	    return FAIL;
 
 	if (ppconst->pp_used == ppconst_used + 2