v6.6.0 spring easing curves for motion tokens

What's Changed

• Improve test coverage by @lukasoppermann in #215
• Spring motion by @lukasoppermann in #214
• Partial PR of blendmode value by @mattpilott in #193

New Contributors

• @mattpilott made their first contribution in #193

Full Changelog: v6.5.0...v6.6.0