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/module_not_imported.fn

@@ -1,4 +0,0 @@
-// expect: to use a module, add  import geometry
-fn main() {
-    v := geometry.Vec(1, 2)
-}