Set placeholder for IntInput (Fixes #8359)

[emunsing]

1-liner fix to add a placeholder value to 8359. I'm not sure how to create a UI test for a placeholder value; happy to create one with help but given that this is a 1-line aesthetic change I don't know if that's needed.

[philippjfr]

Thanks @emunsing, I don't think we really need a UI test, a simply unit test checking that the underlying model inherits the correct placeholder value should be sufficient.

[philippjfr]

def test_int_input_placeholder(document, comm):

    int_input = IntInput(placeholder='Foo", name='IntInput')

    widget = int_input.get_root(document, comm=comm)

    assert widget.active == "Foo"
    
    int_input.placeholder = "Bar"
    assert widget.placeholder == "Bar"