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/control.fn

@@ -1,42 +0,0 @@
-fn classify(int n) -> str {
-    if n < 0 {
-        return "negative"
-    } else if n == 0 {
-        return "zero"
-    } else {
-        return "positive"
-    }
-}
-
-fn main() {
-    print(classify(-5), classify(0), classify(5))
-
-    total := 0
-    for i in 0..100 {
-        if i % 2 == 0 {
-            continue
-        }
-        if i > 10 {
-            break
-        }
-        total += i
-    }
-    print("odd sum below 10:", total)
-
-    n := 0
-    while true {
-        n += 1
-        if n == 5 {
-            break
-        }
-    }
-    print("n:", n)
-
-    for i in 3..3 {
-        print("never")
-    }
-    {
-        inner := "blocks have their own scope"
-        print(inner)
-    }
-}