vim-patch:9.1.1074: Strange error when heredoc marker starts with "trim" (#32317)

Problem:  Strange error when heredoc marker starts with "trim".
Solution: Check for whitespace after "trim" or "eval" (zeertzjq)

For :python3 etc., a heredoc marker that starts with a lower-case letter
is valid, and when it starts with "trim" it works in a script but not in
a function, and this PR makes it works in a function.
For :let, a heredoc marker that starts with a lower-case letter is not
valid, but when it starts with "trim" or "eval" the error can be a bit
confusing in a function, and this PR make it less confusing.

closes: vim/vim#16574

449c2e5454

test/old/testdir/test_python3.vim

@@ -284,7 +284,10 @@ s+='B'
   python3 << trim eof
     s+='E'
   eof
-  call assert_equal('ABCDE', pyxeval('s'))
+  python3 << trimm
+s+='F'
+trimm
+  call assert_equal('ABCDEF', pyxeval('s'))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab