CSS @layer support to override existing presets

[fourdim]

Is your feature request related to a problem? Please describe.
Sometimes users need to add !important to override the styles that already exist in presets.

Describe the solution you'd like
https://developer.mozilla.org/en-US/docs/Web/CSS/@layer
Then we can put presets in the lower layer while user defined CSS in the higher layer.
User defined CSS will automatically override presets so that they don't need to worry about fighting the slidev's presets in terms of selector strength.

Describe alternatives you've considered
None.