[@mantine/prism] Prism: use higher specificity for pre element margin

[mrbnclt]

This fixes #4429

Wrapping Prism within a TypographyStylesProvider that has a selector with the margin values:

<class> pre: {
  marginTop: theme.spacing.md,
  marginBottom: theme.spacing.md
}

has a higher specificity than Prism's selector with the margin set to 0

code: {
  .......
  marginTop: 0,
  marginBottom: 0
}

this causes the margin styles from TypographyStylesProvider to be utilized by Prism's pre element and causes the copy button to move higher than it's normal position.

[rtivital]

Thanks!