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/guess.fn

@@ -1,27 +0,0 @@
-// Guess the number: input, loops and conversions.
-import "stdlib.h"
-import "time.h"
-
-fn main() {
-    srand(u32(time(null)))
-    secret := rand() % 100 + 1
-    tries := 0
-    print("I'm thinking of a number from 1 to 100.")
-    while true {
-        answer := input("Your guess: ")
-        if answer.len == 0 {
-            print("Bye!")
-            return
-        }
-        guess := int(answer)
-        tries += 1
-        if guess < secret {
-            print("Higher!")
-        } else if guess > secret {
-            print("Lower!")
-        } else {
-            print("Yes! It was", secret, "- you needed", tries, "tries.")
-            return
-        }
-    }
-}