feat: convert one stop gradients plugin

[SethFalco]

Detects if a redundant linearGradient or radialGradient is used with only a single stop, which effectively means a solid color.

If this is found, just remove the gradient and replace references to it with the color of the first and only stop defined.

Metrics

I've tested this with the SVG provided in the issue, everything looks identical. Note convertOneStopGradients is only enabled in the this branch row, since it didn't exist before.

SVG	Time	% reduced	final size
Original	N/A	N/A	1.74 KiB
v3.0.2	18 ms	50.6%	0.859 KiB
main (bd750ce)	16 ms	50.6%	0.859 KiB
this branch	19 ms	67.2%	0.571 KiB

SVG	Time	% reduced	final size
Original	N/A	N/A	15060.532 KiB
v3.0.2	14915 ms	31.4%	10326.444 KiB
main (bd750ce)	14641 ms	31.4%	10326.567 KiB
this branch	14478 ms	31.4%	10326.567 KiB

Related

• Closes Remove one-stop gradients #568

Note

• When testing, I temporarily made this a default plugin, so it was being used for yarn run test and yarn run test-regression, with everything passing. 👍🏽