patch 8.1.1116: cannot enforce a Vim script style

Problem:    Cannot enforce a Vim script style.
Solution:   Add the :scriptversion command. (closes #3857)

src/testdir/test_eval_stuff.vim

@@ -123,3 +123,39 @@ func Test_string_concatenation()
   let a..=b
   call assert_equal('ab', a)
 endfunc
+
+scriptversion 2
+func Test_string_concat_scriptversion2()
+  let a = 'a'
+  let b = 'b'
+
+  call assert_fails('echo a . b', 'E15:')
+  call assert_fails('let a .= b', 'E985:')
+  call assert_fails('let vers = 1.2.3', 'E15:')
+
+  if has('float')
+    let f = .5
+    call assert_equal(0.5, f)
+  endif
+endfunc
+
+scriptversion 1
+func Test_string_concat_scriptversion1()
+  let a = 'a'
+  let b = 'b'
+
+  echo a . b
+  let a .= b
+  let vers = 1.2.3
+  call assert_equal('123', vers)
+
+  if has('float')
+    call assert_fails('let f = .5', 'E15:')
+  endif
+endfunc
+
+func Test_scriptversion()
+  call writefile(['scriptversion 9'], 'Xversionscript')
+  call assert_fails('source Xversionscript', 'E999:')
+  call delete('Xversionscript')
+endfunc