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/ptr_value_field.fn

@@ -1,23 +0,0 @@
-struct Node {
-    int value
-    ptr[Node] next
-}
-
-struct Box {
-    int size
-}
-
-fn main() {
-    last := new(Node(value: 2, next: null))
-    first := Node(value: 1, next: last)
-    p := addr(first)
-    print(p.value, p.next.value)
-    p.value = 10
-    print(first.value)
-    b := Box(3)
-    q := addr(b)
-    print(q.value, q.size)
-    q.value = Box(7)
-    print(b)
-    free(last)
-}