patch 8.2.2951: Vim9: cannot use heredoc for :python, :lua, etc.

Problem:    Vim9: cannot use heredoc in :def function for :python, :lua, etc.
Solution:   Concatenate the heredoc lines and pass them in the ISN_EXEC_SPLIT
            instruction.

src/testdir/test_vim9_func.vim

@@ -2758,5 +2758,33 @@ def Test_closing_brace_at_start_of_line()
   call CheckDefAndScriptSuccess(lines)
 enddef
 
+if has('python3')
+  def Test_python3_heredoc()
+    py3 << trim EOF
+      import vim
+      vim.vars['didit'] = 'yes'
+    EOF
+    assert_equal('yes', g:didit)
+
+    python3 << trim EOF
+      import vim
+      vim.vars['didit'] = 'again'
+    EOF
+    assert_equal('again', g:didit)
+  enddef
+endif
+
+" This messes up syntax highlight, keep near the end.
+if has('lua')
+  def Test_lua_heredoc()
+    g:d = {}
+    lua << trim EOF
+        x = vim.eval('g:d')
+        x['key'] = 'val'
+    EOF
+    assert_equal('val', g:d.key)
+  enddef
+endif
+
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker