runtime(vim): Update base-syntax, improve :let-heredoc highlighting

The end marker must appear on line of its own without any trailing
whitespace.

Whitespace is incorrectly allowed before all end markers.  Limiting this
only to heredocs where "trim" was specified, and with the correct
indent, is currently an intractable problem given that contained syntax
groups (in this case :let) cannot be limited to start patterns.

Highlight interpolated expressions when "eval" is specified.

cloess: #15511

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_ex_function_fold.vim

@@ -193,10 +193,9 @@ function Foo()
 endfunction
 
 function Foo()
-  let x =<< END
+  let x =<< trim END
     endfunction
   END
-
 endfunction
 
 function Foo()