Use min and max values in Storybook breakpoints

[vdepizzol]

Instead of using single-value options in Storybook's viewport sizes, add support for minimum and maximum values for each of Primer's breakpoints, allowing for verifying exactly how the components and patterns behave in each breakpoint extreme (and whether it correctly adjusts as the breakpoint changes).

A few notes:

• I'm using xs to define anything smaller than $width-sm, as documented here:
  https://primer.style/css/support/breakpoints#breakpoint-variables

• I'm using 375px as the minimum xs size as a reference to the minimum viewport size we should support.

/cc @langermank

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Latest commit: ab02070

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[langermank]

Cool! 🚀