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>

examples/todo.fn

@@ -1,53 +0,0 @@
-// A tiny to-do list: structs, arrays, strings and files.
-
-struct Task {
-    str text
-    bool done = false
-}
-
-fn show([]Task tasks) {
-    if tasks.len == 0 {
-        print("(nothing to do)")
-    }
-    for i in 0..tasks.len {
-        mark := " "
-        if tasks[i].done {
-            mark = "x"
-        }
-        print("[" + mark + "]", i + 1, tasks[i].text)
-    }
-}
-
-fn save([]Task tasks, str path) {
-    []str lines
-    for t in tasks {
-        lines.push(str(t.done) + "|" + t.text)
-    }
-    write_file(path, lines.join("\n"))
-}
-
-fn load(str path) -> []Task {
-    []Task tasks
-    if !file_exists(path) {
-        return tasks
-    }
-    for line in read_file(path).split("\n") {
-        if line.len == 0 {
-            continue
-        }
-        parts := line.split("|")
-        tasks.push(Task(text: parts[1], done: parts[0] == "true"))
-    }
-    return tasks
-}
-
-fn main() {
-    path := "todo.txt"
-    tasks := load(path)
-    tasks.push(Task(text: "learn Finch"))
-    tasks.push(Task(text: "write a game"))
-    tasks[0].done = true
-    show(tasks)
-    save(tasks, path)
-    print("saved", tasks.len, "tasks to", path)
-}