patch 8.2.0421: interrupting with CTRL-C does not always work

Problem:    Interrupting with CTRL-C does not always work.
Solution:   Recognize CTRL-C while modifyOtherKeys is set.

src/testdir/test_vim9_script.vim

@@ -942,15 +942,15 @@ def Test_while_loop()
   assert_equal('1_3_', result)
 enddef
 
-" def Test_interrupt_loop()
-"   let x = 0
-"   while 1
-"     x += 1
-"     if x == 100
-"       feedkeys("\<C-C>", 'L')
-"     endif
-"   endwhile
-" enddef
+def Test_interrupt_loop()
+  let x = 0
+  while 1
+    x += 1
+    if x == 100
+      feedkeys("\<C-C>", 'Lt')
+    endif
+  endwhile
+enddef
 
 def Test_substitute_cmd()
   new