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/fail/mixed_signs.fch

@@ -0,0 +1,6 @@
+// expect: can't use '+' on i32 and u32
+fn main() {
+    i32 a = 1
+    u32 b = 2
+    print(a + b)
+}