patch 8.1.2341: not so easy to interrupt a script programatically

Problem:    Not so easy to interrupt a script programatically.
Solution:   Add the interrupt() function. (Yasuhiro Matsumoto, closes #2834)

src/testdir/test_interrupt.vim

@@ -0,0 +1,27 @@
+" Test behavior of interrupt()
+
+let s:bufwritepre_called = 0
+let s:bufwritepost_called = 0
+
+func s:bufwritepre()
+  let s:bufwritepre_called = 1
+  call interrupt()
+endfunction
+
+func s:bufwritepost()
+  let s:bufwritepost_called = 1
+endfunction
+
+func Test_interrupt()
+  new Xfile
+  let n = 0
+  try
+    au BufWritePre Xfile call s:bufwritepre()
+    au BufWritePost Xfile call s:bufwritepost()
+    w!
+  catch /^Vim:Interrupt$/
+  endtry
+  call assert_equal(1, s:bufwritepre_called)
+  call assert_equal(0, s:bufwritepost_called)
+  call assert_equal(0, filereadable('Xfile'))
+endfunc