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/types.fn

@@ -1,31 +0,0 @@
-// Sized numbers and pointers.
-
-fn double(ptr[int] p) {
-    p.value = p.value * 2
-}
-
-fn main() {
-    // everyday: int and float. For bytes and hardware: exact sizes.
-    u8 small = 255
-    i32 points = -1000
-    big := u64(0xFFFF_FFFF)
-    f32 half = 0.5
-
-    small += 1                       // u8 wraps around, like real hardware
-    print("u8 255 + 1 =", small)
-    print("points:", points, "big:", big, "half:", half)
-
-    i64 wide = points                // a smaller type fits into a bigger one by itself
-    print("wide:", wide)
-
-    // pointers: addr(x) gives you where x lives, p.value is what is there
-    int x = 21
-    p := addr(x)
-    double(p)
-    print("x is now", x)
-
-    ptr[int] nothing = null
-    if nothing == null {
-        print("nothing points nowhere")
-    }
-}