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>

tests/run/pointers.fn

@@ -1,25 +0,0 @@
-fn double(ptr[int] p) {
-    p.value = p.value * 2
-}
-
-fn swap(ptr[int] a, ptr[int] b) {
-    t := a.value
-    a.value = b.value
-    b.value = t
-}
-
-fn main() {
-    x := 21
-    double(addr(x))
-    print(x)
-    a := 1
-    b := 2
-    swap(addr(a), addr(b))
-    print(a, b)
-    p := addr(a)
-    pp := addr(p)
-    pp.value.value = 99
-    print(a)
-    ptr[int] none = null
-    print(none == null, p != null, none)
-}