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/functions.fn

@@ -1,28 +0,0 @@
-fn fib(int n) -> int {
-    if n < 2 {
-        return n
-    }
-    return fib(n - 1) + fib(n - 2)
-}
-
-fn later() -> int {
-    return defined_below()
-}
-
-fn defined_below() -> int {
-    return 42
-}
-
-fn greet(str who) {
-    if who == "" {
-        return
-    }
-    print("hi", who)
-}
-
-fn main() {
-    print(fib(20))
-    print(later())
-    greet("")
-    greet("finch")
-}