patch 9.1.1074: Strange error when heredoc marker starts with "trim"

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: #16574

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_let.vim

@@ -435,6 +435,24 @@ func Test_let_heredoc_fails()
     call assert_report('Caught exception: ' .. v:exception)
   endtry
 
+  try
+    let v =<< trim trimm
+    trimm
+    call assert_report('No exception thrown')
+  catch /E221:/
+  catch
+    call assert_report('Caught exception: ' .. v:exception)
+  endtry
+
+  try
+    let v =<< trim trim evall
+    evall
+    call assert_report('No exception thrown')
+  catch /E221:/
+  catch
+    call assert_report('Caught exception: ' .. v:exception)
+  endtry
+
   let text =<< trim END
   func WrongSyntax()
     let v =<< that there