feat: Add token definitions and lexer logic.

This commit adds the `Token` and `TokenKind` definitions in `src/token.rs`,
in `src/lexer.rs` I've added the `Lexer` logic.

squash

examples/fibonacci.bky

@@ -0,0 +1,7 @@
+fn fib(n: u64) -> u64 {
+    if n < 2 {
+        return n;
+    }
+
+    return fib(n - 1) + fib(n - 2);
+}