feat: add flat_tree allgorithm

app/src/routes/test/+page.svelte

@@ -0,0 +1,20 @@
+<script lang="ts">
+  import { decode, encode } from "$lib/helpers/flat_tree";
+
+  const input = [5, [6, 1], [7, 2, [5, 1]]];
+  // const input = [5, [], [6, []], []];
+  // const input = [52];
+
+  console.log("INPUT");
+  console.log(input);
+
+  const encoded = encode(input);
+  console.log("ENCODED");
+  console.log(encoded);
+
+  const decoded = decode(encoded);
+  console.log("DECODED");
+  console.log(decoded);
+
+  console.log("EQUALS", JSON.stringify(input) === JSON.stringify(decoded));
+</script>