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.

examples/hello-world.bky

@@ -0,0 +1,5 @@
+extern puts(text: *char);
+
+fn main() {
+    puts("Hello, World!");
+}