patch 9.0.2157: Vim9: incorrectly parses :def func definitions

patch 9.0.2157: Vim9: incorrectly parses :def func definitions

Problem:  Vim9: incorrectly parses :def func definitions
Solution: check for more context when parsing function args

Signed-off-by: Christian Brabandt <cb@256bit.org>
Incorrectly parses def function definitions

Vim currently allows to define the following vim9 function:
  def Func(f=
  )
  enddef

It currently thinks a Lambda is following the `=` but it doesn't check,
that there is actually an expression following. So when such a think is
encountered, remember that an expression should be following.

If no expression is coming in the next few lines, fail parsing the
function arguments, which will Vim no longer accept such a function.

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