fix(ui): use a linear gradient ramp glyph for the Gradient tool icon (#4172) (#4335)

* fix(ui): use a linear gradient ramp glyph for the Gradient tool icon (#4172)

* test(ui): verify Gradient tool icon rasterizes a left-to-right ramp (#4172)

assets/icons/gradient.svg

@@ -0,0 +1,19 @@
+<svg
+  xmlns="http://www.w3.org/2000/svg"
+  width="24"
+  height="24"
+  viewBox="0 0 24 24"
+  fill="none"
+  stroke="currentColor"
+  stroke-width="2"
+  stroke-linecap="round"
+  stroke-linejoin="round"
+>
+  <defs>
+    <linearGradient id="g" x1="3" y1="0" x2="20" y2="0" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="currentColor" stop-opacity="0.9" />
+      <stop offset="100%" stop-color="currentColor" stop-opacity="0" />
+    </linearGradient>
+  </defs>
+  <rect width="18" height="16" x="3" y="4" rx="2" fill="url(#g)" />
+</svg>