runtime(vim): Distinguish Vim9 constructor definitions from the :new ex command ...

runtime(vim): Distinguish Vim9 constructor definitions from the :new ex command (#14050)

With the arrival of Vim9 classes, the syntax must allow for
_new_ constructors; multiple constructor definitions are
supported for a class, provided distinct suffix-names are
used.  Currently, the defined constructors match either
vimCommand or vimFunctionError (for any newBar).

For example:
------------------------------------------------------------
vim9script

class Foo
    def new()
    enddef

    def newBar()
    enddef
endclass
------------------------------------------------------------

Since every constructor is required to bear a lower-cased
_new_ prefix name, it should suffice to distinguish them
from functions, and so there are no new highlight or syntax
groups introduced.

Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: h-east <h.east.727@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>