chore: add supabase icon (#28327)

Adds the Supabase icon to the built-in icons for use with external auth
and modules.

- Adds `site/static/icon/supabase.svg` (official Supabase logo)
- Registers icon in `site/src/theme/icons.json`

The icon is full-color with good contrast on both light and dark
backgrounds, so no special handling in `externalImages.ts` is needed.

site/src/theme/icons.json

@@ -134,6 +134,7 @@
 	"rustrover.svg",
 	"slack.svg",
 	"sourcegraph-amp.svg",
+	"supabase.svg",
 	"swift.svg",
 	"tasks.svg",
 	"tensorflow.svg",

site/static/icon/supabase.svg

@@ -0,0 +1,15 @@
+<svg width="256" height="256" viewBox="0 0 256 256" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M148.867 249.758C141.986 255.922 128.837 252.678 128.681 242.777L126.394 90.696H228.994C247.574 90.696 257.959 111.104 246.397 124.881L148.867 249.758Z" fill="url(#paint0_linear)"/>
+<path d="M148.867 249.758C141.986 255.922 128.837 252.678 128.681 242.777L126.697 90.696H228.994C247.574 90.696 257.959 111.104 246.397 124.881L148.867 249.758Z" fill="url(#paint1_linear)" fill-opacity="0.2"/>
+<path d="M107.133 4.691C114.014 -1.473 127.163 1.771 127.319 11.672L128.161 163.753H26.817C8.237 163.753 -2.148 143.345 9.414 129.568L107.133 4.691Z" fill="#3ECF8E"/>
+<defs>
+<linearGradient id="paint0_linear" x1="126.697" y1="124.503" x2="217.786" y2="162.717" gradientUnits="userSpaceOnUse">
+<stop stop-color="#249361"/>
+<stop offset="1" stop-color="#3ECF8E"/>
+</linearGradient>
+<linearGradient id="paint1_linear" x1="86.340" y1="69.279" x2="127.894" y2="147.440" gradientUnits="userSpaceOnUse">
+<stop/>
+<stop offset="1" stop-opacity="0"/>
+</linearGradient>
+</defs>
+</svg>