feat: create Gradient and GradientImmu

[rafamizes]

Closes #83

[codecov]

Codecov Report

Merging #86 (43d54b3) into master (ec85fb9) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 43d54b3 differs from pull request most recent head 2a59029. Consider uploading reports for the commit 2a59029 to get more accurate results

@@            Coverage Diff            @@
##            master       #86   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           78        78           
  Lines          326       326           
=========================================
  Hits           326       326           

Impacted Files	Coverage Δ
lib/src/palette_rgb.dart	100.00% <ø> (ø)
lib/src/swatch_base.dart	100.00% <ø> (ø)
lib/src/gradient_immu.dart	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec85fb9...2a59029. Read the comment docs.

[rafamizes]

@rultor merge.

[rultor]

@rultor merge.

@rafamizes OK, I'll try to merge now. You can check the progress of the merge here

[rultor]

@rultor merge.

@rafamizes Done! FYI, the full log is here (took me 3min)