[orx-shade-styles] Add HemisphereLight shader and demo example

Introduce the `HemisphereLight` shader for smooth gradient lighting based on surface orientation relative to a light direction. Add a demo featuring the Suzanne model to showcase the shader in action.

orx-shade-styles/src/commonMain/kotlin/spatial/HemisphereLight.kt

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+package org.openrndr.extra.shadestyles.spatial
+
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.ShadeStyle
+import org.openrndr.math.Vector3
+
+class HemisphereLight: ShadeStyle() {
+    var upColor: ColorRGBa by Parameter()
+    var downColor: ColorRGBa by Parameter()
+    var lightDirection: Vector3 by Parameter()
+
+    init {
+        lightDirection = -Vector3.UNIT_Y
+        upColor = ColorRGBa.WHITE
+        downColor = ColorRGBa.BLACK
+        fragmentTransform = """
+            vec3 n = normalize(v_worldNormal);
+            float d = dot(n, p_lightDirection) * 0.5 + 0.5;
+            x_fill = mix(p_upColor, p_downColor, d);
+        """.trimIndent()
+    }
+}

orx-shade-styles/src/jvmDemo/kotlin/spatial/DemoHemisphere01.kt

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+package spatial
+
+import org.openrndr.WindowMultisample
+import org.openrndr.application
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.DrawPrimitive
+import org.openrndr.extra.camera.Orbital
+import org.openrndr.extra.color.presets.SADDLE_BROWN
+import org.openrndr.extra.objloader.loadOBJasVertexBuffer
+import org.openrndr.extra.shadestyles.spatial.HemisphereLight
+import org.openrndr.math.Vector3
+
+fun main() {
+    application {
+        configure {
+            width = 720
+            height = 720
+            multisample = WindowMultisample.SampleCount(8)
+        }
+
+        program {
+            val obj = loadOBJasVertexBuffer("demo-data/obj-models/suzanne/Suzanne.obj")
+
+            extend(Orbital()) {
+                eye = Vector3(0.0, 0.0, 2.0)
+            }
+            extend {
+                drawer.shadeStyle = HemisphereLight().apply {
+                    downColor = ColorRGBa.SADDLE_BROWN
+                }
+                drawer.vertexBuffer(obj, DrawPrimitive.TRIANGLES)
+            }
+        }
+    }
+}