Make the style of ui.separator be different in different orientation

[python-and-novella]

Motivation

Fix #5270, where the style of ui.separator().props('vertical') doesn't work as expected in NiceGUI 3.0.0. above.

Implementation

change

.nicegui-separator {
  width: 100%;
}

into

.q-separator--vertical.nicegui-separator {
  width: 1px;
}
.q-separator--horizontal.nicegui-separator {
  width: 100%;
}

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).

Test

Testing it by:

from nicegui import ui

def index():
    with ui.column():
        ui.button('1')
        ui.separator()
        ui.button('2')
    with ui.row():
        ui.button('1')
        ui.separator().props('vertical')
        ui.button('2')

ui.run(
    root=index
)

It should work as following:

[python-and-novella]

I'm sorry for that I have no time to write unit test. And I have no idea of writing unit test about this.
There isn't a unit test about ui.separator, I think the unit test is necessary.

[evnchn]

I don't think that's how the tests are supposed to work. (You're more like checking whether the styles which we applied are applied, which they sure are, from the tests which tests that part)

Maybe you can test the aspect ratio of the rendered divider, instead.

[python-and-novella]

It is difficult for me without learning carefully. I will try it when I am free.
I am glad for that if someone can make unit test complete.

[evnchn]

Honestly I am not that good at pytest either, but looking back I think we can get away with it.

I didn't have any tests in #5266 either.

[falkoschindler]

Thanks for the pull request, @python-and-novella!
I haven't had the chance to think about it, yet. But either way, I don't think pytests are necessary. We don't check the visual appearance for most elements, so we don't need to start here. If we would start checking aspect ratios, I would also need to check color, shadow, exact dimensions etc. to prevent regressions. This isn't feasible.