feat: add support for %/px stop positions

[gluck]

• negative (and >1) values ignored because unsupported in figma (could probably rescale the gradient vector to emulate them)
• em (and alike) units unsupported
• add frame width/height as parameters to compute px positions (will probably be needed as well to fix angular gradients #9 )
• fixes #10

[gluck]

background: linear-gradient(45deg, rgb(236, 72, 153) 25%, rgb(239, 68, 68) 25%, rgb(239, 68, 68) 75%, rgb(234, 179, 8) 75%);
background: linear-gradient(45deg, rgb(236, 72, 153) 25px, rgb(239, 68, 68) 25px, rgb(239, 68, 68) 75px, rgb(234, 179, 8) 75px);

https://codepen.io/gluckies/pen/gOzyQJe

[yagudaev]

Yay! 🥳

Thanks for the PR, take a look at it now. Really appreciate it 😁

[yagudaev]

Excellent work! Added a few housekeeping things and tested it 😁.

You are right about negative values, and yet it will require scaling the gradient. Note that stops above 100% will also require scaling.