Rename the Finch file extension from .fn to .fch

.fn is already used by other tools (e.g. Fennel). Modules are now looked up as name.fch;
a leftover name.fn gets a clear message telling you to rename it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/window.fn

@@ -1,34 +0,0 @@
-// A window that slowly changes color. Needs GLFW and OpenGL
-// (on Nix they are in shell.nix: run this inside nix-shell).
-import "GLFW/glfw3.h"
-import "GL/gl.h"
-import "math.h"
-link "glfw"
-link "GL"
-
-fn main() {
-    if glfwInit() == 0 {
-        print("could not start GLFW")
-        return
-    }
-
-    window := glfwCreateWindow(800, 600, "Finch", null, null)
-    if window == null {
-        print("could not open a window")
-        glfwTerminate()
-        return
-    }
-    glfwMakeContextCurrent(window)
-
-    while glfwWindowShouldClose(window) == 0 {
-        t := glfwGetTime()
-        glClearColor(f32(sin(t) * 0.5 + 0.5), 0.3, 0.6, 1.0)
-        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
-
-        glfwSwapBuffers(window)
-        glfwPollEvents()
-    }
-
-    glfwDestroyWindow(window)
-    glfwTerminate()
-}