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/defer.fn

@@ -1,20 +0,0 @@
-fn work(int n) -> int {
-    defer print("cleanup", n)
-    if n > 1 {
-        return n * 10
-    }
-    defer print("only when small")
-    return n
-}
-
-fn main() {
-    print(work(1))
-    print(work(2))
-    for i in 0..3 {
-        defer print("end of round", i)
-        if i == 1 {
-            continue
-        }
-        print("round", i)
-    }
-}