Change PostCSS to v8

[arturparkhisenko]

• Changed postcss dependency version to 8 🆕, plugin migration guide.
• Changed Node.js version to 10. Note: PostCSS v8 dropped Node.js 6.x, 8.x, 11.x, and 13.x versions support.

Closes #12

[arturparkhisenko]

@larsenwork do you know who is a maintainer there who can review this PR?

[jonnitto]

@larsenwork Any news on this?

[arturparkhisenko]

@larsenwork can you please review it?

And as a part of the Hacktoberfest OSS initiative, I wanted to ask you to add the label hacktoberfest-accepted if that PR will be accepted and merged.

[arturparkhisenko]

Last year I did open it when I hoped to resolve this dependency thing, but because of waiting, we ended writing our own function based on multiple sources. If it'll help anyone:

• https://css-tricks.com/easing-linear-gradients/ from the author of this repo & plugin @larsenwork
• [css-images] Add easing functions to color stops w3c/csswg-drafts#1332 a good draft
• https://easings.net/

[thomasaull]

@arturparkhisenko Would you be willing to share your solution?

[arturparkhisenko]

@thomasaull Hi, unfortunately, I cannot share it, the project is not open source, but what can help to create the solution:

• https://css-tricks.com/easing-linear-gradients/#comment-1608608
• https://codepen.io/jmkII/pen/pParwj?editors=0110

Those two code parts are a bit simpler to read than the plugin.
In addition to the just easing function, we did write the ease-in-out-sine function for steps progress (so it's not hardcoded and the amount of steps is a variable) and implemented support for transparency. Last but not least if it's a JS function then chroma.js it's a library that might help a lot. We did write our own using SASS and relied on built-ins.

[Ambient-Impact]

@arturparkhisenko Glad to see some development on this!