patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)

src/testdir/test_tcl.vim

@@ -67,11 +67,11 @@ func Test_vim_buffer()
   " Test ::vim::buffer list
   call assert_equal('2',    TclEval('llength [::vim::buffer list]'))
   call assert_equal(b1.' '.b2, TclEval('::vim::buffer list'))
-  tcl <<EOF
+  tcl << trim EOF
     proc eachbuf { cmd } {
       foreach b [::vim::buffer list] { $b command $cmd }
     }
-EOF
+  EOF
   tcl eachbuf %s/foo/FOO/g
   b! Xfoo1
   call assert_equal(['FOObar'], getline(1, '$'))
@@ -680,3 +680,22 @@ func Test_set_cursor()
   normal j
   call assert_equal([2, 5], [line('.'), col('.')])
 endfunc
+
+" Test for different syntax for ruby heredoc
+func Test_tcl_heredoc()
+  tcl << END
+::vim::command {let s = "A"}
+END
+  tcl <<
+::vim::command {let s ..= "B"}
+.
+  tcl << trim END
+    ::vim::command {let s ..= "C"}
+  END
+  tcl << trim
+    ::vim::command {let s ..= "D"}
+  .
+  call assert_equal('ABCD', s)
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab