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/tour.fn

@@ -1,62 +0,0 @@
-// A quick tour of Finch.
-
-fn add(int a, int b) -> int {
-    return a + b
-}
-
-fn fib(int n) -> int {
-    if n < 2 {
-        return n
-    }
-    return fib(n - 1) + fib(n - 2)
-}
-
-fn isEven(int n) -> bool {
-    return n % 2 == 0
-}
-
-fn greet(str name) {
-    if name == "Finch" {
-        print("Hi, it's me!")
-    } else {
-        print("Hi,", name)
-    }
-}
-
-fn main() {
-    // two ways to make a variable
-    int age = 10
-    name := "Ola"          // type is guessed: str
-    pi := 3.14             // float
-    letter := 'F'          // char
-
-    print(name, "is", age, "years old")
-    print("2 + 3 =", add(2, 3))
-    print("pi * 2 =", pi * 2)
-    print("first letter:", letter)
-    print("10 is even?", isEven(10))
-
-    greet("Finch")
-    greet(name)
-
-    // loops
-    for i in 0..10 {
-        if i == 3 {
-            continue
-        }
-        if i == 7 {
-            break
-        }
-        print("i =", i)
-    }
-
-    count := 3
-    while count > 0 {
-        print("countdown", count)
-        count -= 1
-    }
-
-    print("fib(30) =", fib(30))
-    print("7 / 2 =", 7 / 2, "and as float:", float(7) / 2)
-    print("int(9.99) =", int(9.99))
-}