init: Add Token definition and Tokenizer logic.

example/main.bky

@@ -0,0 +1,8 @@
+/// This function computes the n-th value of the fibbonacci sequence.
+fn fib(n: u64): u64 {
+    if n < 2 {
+        return n;
+    }
+
+    return fib(n - 1) + fib(n - 2);
+}