Move gradient picking into the color picker (#1778)

* Gradient picker

* Fix up color picker layout CSS problems

* Begin hooking up SpectrumInput for gradient in the ColorPicker

* Working gradient picking on the frontend only

* Plumb FillColorChoice into the backend

* Hook everything else up, just with a weird bug remaining

* Fix some svelty reactivity issues

* Add and remove stops

* Cleanup

* Rename type

* Fill node document format upgrading

* Fix lint

* Polish the color picker UX and fix a bug

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Co-authored-by: 0hypercube <0hypercube@gmail.com>

frontend/assets/icon-12px-solid/swap-horizontal.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 12 12">
+	<path d="M9,7l3-3L9,1v2H5v2h4V7z M0,8l3,3V9h4V7H3V5L0,8z" />
+</svg>