feat: eased gradients

[ricokahler]

This PR is still a work in progress but I wanted to pause here for feedback. For the most part, this PR is as planned from #441 but instead of changing the cubic bezier functions to number array's, I opted to parse them via regex since that's how colors are read anyway.

Currently, this PR just adds a few functions that will eventually power eased gradients. The main one is getScale which will eventually be imported in linearGradient and radialGradient to enable eased gradients via the color stop array (with no breaking API change).

getScale accepts color stops with timing functions in between them. Refer to the tests for usage.

TODO:

• [ ] getScale needs to be integrated with linearGradient and radialGradient
• [ ] Errors need to use the PolishedError with new error messages added to the docs.
• [ ] One gotcha I ran into is that mix doesn't mix transparent and non-transparent colors. This restriction should be removed if we want to ease black with transparent
• [ ] Add flow types (I don't know flow, only typescript)
• [ ] Update docs
• [ ] Fix math. Taking a second look, I think the math is wrong for the cubic bezier curve so the equation needs to be fixed but the the approach is still the same

Screenshots (for getScale)

Here's one with an eased gradient 👇

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Anyway, at this point I think you can see where I'm going with this, and to be honest, I'm not sure I like these changes. It's a lot of code for a relatively niche feature and I'm wondering if you can think of a more simple way to get this done.

Let me know what you think!

cc @bhough

[ricokahler]

I just noticed my branch includes some isValidColor stuff. I don't know how that got in there but ignore it in the diff for now.

[codecov[bot]]

Codecov Report

Merging #523 into version-4-0-release will decrease coverage by 0.94%. The diff coverage is 89.65%.

@@                   Coverage Diff                   @@
##           version-4-0-release     #523      +/-   ##
=======================================================
- Coverage                99.88%   98.94%   -0.95%     
=======================================================
  Files                       87       91       +4     
  Lines                      857      944      +87     
  Branches                   325      344      +19     
=======================================================
+ Hits                       856      934      +78     
- Misses                       1        6       +5     
- Partials                     0        4       +4     

Flag	Coverage Δ
#unittests	98.94% <89.65%> (-0.95%)	:arrow_down:

Impacted Files	Coverage Δ
src/internalHelpers/_parseTimingFunction.js	57.89% <57.89%> (ø)
src/color/getScale.js	94.44% <94.44%> (ø)
src/color/isValidColor.js	100.00% <100.00%> (ø)
src/internalHelpers/_normalizeColorStopArray.js	100.00% <100.00%> (ø)

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[ricokahler]

So I've sat on this for a while and I just don't like this approach. Since there's no easy way to create a timing function, I would advise to not ship eased gradients in v4.