patch 8.1.2038: has('vimscript-4') is always 0

Problem:    has('vimscript-4') is always 0.
Solution:   Add "vimscript-4" to the feature table. (Naruhiko Nishino,
            closes #4941)

src/evalfunc.c

@@ -3771,6 +3771,7 @@ f_has(typval_T *argvars, typval_T *rettv)
 	"vimscript-1",
 	"vimscript-2",
 	"vimscript-3",
+	"vimscript-4",
 	"virtualedit",
 	"visual",
 	"visualextra",

src/testdir/test_eval_stuff.vim

@@ -194,6 +194,7 @@ endfunc
 
 scriptversion 4
 func Test_vvar_scriptversion4()
+  call assert_true(has('vimscript-4'))
   call assert_equal(17, 017)
   call assert_equal(18, 018)
   call assert_equal(64, 0b1'00'00'00)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2038,
 /**/
     2037,
 /**/