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/input.fn

@@ -1,7 +0,0 @@
-fn main() {
-    name := input("Name? ")
-    age := int(input("Age? "))
-    print("Hi", name, "next year you will be", age + 1)
-    rest := input()
-    print("rest:", rest, rest.len)
-}