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/basics.fn

@@ -1,18 +0,0 @@
-fn main() {
-    int a = 7
-    b := 2
-    print(a + b, a - b, a * b, a / b, a % b)
-    print(-a / b, -a % b)
-    x := 1.5
-    print(x * 2, x + 1, 7 / 2.0)
-    print(true && false, true || false, !true)
-    print('A', "text", 1 < 2, 2 <= 2, 3 > 4, 4 >= 4, 1 == 1, 1 != 1)
-    a += 3
-    a -= 1
-    a *= 2
-    a /= 3
-    a %= 4
-    print(a)
-    print()
-    print("tab\there", "quote\"inside")
-}