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/bits.fch

@@ -0,0 +1,9 @@
+fn main() {
+    print(6 & 3, 6 | 3, 6 ^ 3, 1 << 4, 256 >> 2, ~0)
+    print(5 & 1 == 1)
+    u8 flags = 0
+    flags = flags | 4
+    flags = flags | 1
+    print(flags, flags & 4 != 0, flags & 2 != 0)
+    print(-16 >> 2, u8(240) >> 4)
+}