v3.0.0

The two gradient mixins are replaced by one, gradient, which also draws conic
and repeating gradients and blends in a colour space, and text-gradient takes
the same arguments. Both changes break existing calls and both stop the build,
so nothing fails silently. MIGRATION.md has every old call's new form.

• Added: gradient, a linear, radial or conic gradient, plain or repeating,
  written as background-image. The colours come first and everything else has
  a default. $in sets a colour space, such as oklab or "oklch longer hue",
  and the gradient is written once without it first, for a browser that does
  not support colour spaces. A direction takes any angle unit, "to top" or
  var().
• Added: gradientValue, the same gradient as a value, to layer it with an
  image in one background-image or use it as a mask-image or
  border-image.
• Removed: linear-gradient and radial-gradient. Every call to them has a
  gradient call that writes the same gradient; the property is now
  background-image instead of the background shorthand, which also reset
  any background colour set before it.
• Updated: text-gradient takes the colours first, then every argument
  gradient takes, so text can carry a radial, conic or repeating gradient and
  blend in a colour space. A call in the old order stops the build and shows the
  new form. Its output for the same arguments is unchanged.
• Updated: text-gradient's $direction and background-image's
  $filter-direction accept any angle unit, "to top", var() and calc().
• Updated: a gradient with a single colour warns in gradient,
  gradientValue and text-gradient, since browsers before Chrome 135,
  Firefox 136 and Safari 18.4 drop it.

Migration guide: https://github.com/selfishprimate/gerillass/blob/main/MIGRATION.md

Full Changelog: v2.3.1...v3.0.0