break: allow sizes to be themeable

[roydukkey]

@use '@sass-fairy/break' with (
  $horizontal-sizes: (x-small: 0, medium: 680px, large: 960px, x-large: 1400px)
);

Forcing sizes to be definitive values, makes it impossible to created themed breakpoints. It would also be useful to describe sizes are custom properties.

@use '@sass-fairy/break' with (
  $horizontal-sizes: (x-small: 0, medium: var(--breakpoint-m), large: var(--breakpoint-l), x-large: var(--breakpoint-xl))
);

[roydukkey]

Might need to wait on this since custom properties are not supported in media queries. https://bholmes.dev/blog/alternative-to-css-variable-media-queries/