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/raylib.fn

@@ -1,15 +0,0 @@
-// A raylib window with a circle: C structs (Color, Vector2) passed by value.
-// Needs raylib (in shell.nix).
-import "raylib.h"
-link "raylib"
-
-fn main() {
-    InitWindow(800, 450, "Finch")
-    while !WindowShouldClose() {
-        BeginDrawing()
-        ClearBackground(Color(r: 30, g: 30, b: 40, a: 255))
-        DrawCircleV(Vector2(400, 225), 50, Color(255, 136, 0, 255))
-        EndDrawing()
-    }
-    CloseWindow()
-}