'Starter Project': port planet fragment shader to impeller tests (#53362)

This is a modified planet shader from the SimonDev's "GLSL Shaders from Scratch" course on teachables. With permission from Simon:

> 100% use however you want, with or without credit

I choose "with credit".

https://github.com/flutter/engine/assets/1924313/47da62b5-6b6d-47c2-ac9f-f3a33f177a86

Author: jtmcdole

impeller/fixtures/BUILD.gn

@@ -32,6 +32,8 @@ impeller_shaders("shader_fixtures") {
     "instanced_draw.vert",
     "mipmaps.frag",
     "mipmaps.vert",
+    "planet.frag",
+    "planet.vert",
     "sample.comp",
     "sepia.frag",
     "sepia.vert",