fix(menu): allow styling of the box shadow and transform when visible inside of a split pane

[brandyscarney]

Issue number: resolves #21530

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What is the current behavior?

The box-shadow and transform properties cannot be styled on a menu when it is inside of a split pane and visible due to the following:

1. The box-shadow and transform properties are set to none with !important
2. The menu itself is not on "top" of the split pane content so the content hides any box shadow

What is the new behavior?

• Removes all uses of !important in split pane and menu
• Developers will need to change the z-index from 0 to 1 on the ion-menu so the menu will sit on top of the split pane and show the box-shadow

Does this introduce a breaking change?

• Yes
• No

Other information

I considered putting the test for this in split-pane but I thought since it was styling the ion-menu it should go in menu. I can move this if others think it makes more sense in split-pane.