refactor(css): wrap styles in @layer

[joshblack]

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[lukasoppermann]

SelectPanel

@joshblack This is not an issue in SelectPanel but actually comes from a larger space between the label and the segementedControl.

The SegementedControl had a margin top 4px before which is missing now.

It was coming from this class:

.prc-FormControl-ControlVerticalLayout-wOp8f>:not(label)+*, .prc-FormControl-ControlVerticalLayout-wOp8f[data-has-label]>*+*

[lukasoppermann]

TextInput

The new TextInput is 2px wider. The actual input part is 2px wide (not the button or something). I have no idea where this is coming from.

ButtonGroup

The border radius on the buttons is not removed.

The button layer overwrites the reset in the buttonGroup layer:

If I change the layer from ButtonGroup to Button it works. I guess it is specificity because we have different layers. Is it really necessary to have one layer per component? Wouldn't primer.react.components be enough?