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/memory.fn

@@ -1,98 +0,0 @@
-struct Node {
-    str name
-    ptr[Node] next
-    []str tags
-}
-
-fn build(int n) -> []str {
-    []str out
-    out.resize(n)
-    return out
-}
-
-fn firstLong([]str words) -> str {
-    for w in words {
-        tmp := w + "!"
-        if w.len > 3 {
-            return tmp
-        }
-    }
-    return "none"
-}
-
-fn loops() -> int {
-    count := 0
-    for i in 0..10 {
-        s := "item " + str(i)
-        if i == 2 {
-            continue
-        }
-        if i == 6 {
-            break
-        }
-        count += s.len
-    }
-    while true {
-        t := "x" + "y"
-        if t.len == 2 {
-            break
-        }
-    }
-    return count
-}
-
-fn deferred() -> str {
-    log := "start"
-    defer print("deferred ran, log was:", log)
-    log = log + " middle"
-    names := ["a", "b"]
-    defer print("names:", names)
-    return log + " end"
-}
-
-fn main([]str args) {
-    print(args.len > 0)
-    print(firstLong(["hi", "hello", "hey"]))
-    print(firstLong(["a"]))
-    print(loops())
-    print(deferred())
-
-    s := "ab"
-    s = s + s
-    s = s
-    print(s)
-
-    list := ["one", "two", "three"]
-    for w in list {
-        if w == "two" {
-            list.push("four")
-        }
-    }
-    print(list)
-    last := list.pop()
-    print(last, list)
-
-    head := new(Node(name: "first", tags: ["t1"]))
-    head.next = new(Node(name: "second"))
-    print(head.name, head.next.name, head.tags)
-    n := head
-    while n != null {
-        print("node", n.name)
-        n = n.next
-    }
-    free(head.next)
-    free(head)
-
-    []str words
-    for i in 0..3 {
-        words.push("w" + str(i))
-    }
-    words.clear()
-    words.resize(2)
-    print(words, words.len)
-    print([1, 2, 3].slice(1, 3), "abc".bytes())
-    x := ["keep"]
-    y := x
-    x[0] = "changed"
-    print(x, y)
-}