perf: add string flattening to reduce retained memory (node only)

add a memory footprint benchmark

src/parser/flatten/browser.ts

@@ -0,0 +1,3 @@
+export function flatten (str: string) {
+  return str
+}

src/parser/flatten/node.ts

@@ -0,0 +1,10 @@
+/**
+ * This function forces a string to be re-instantiated in memory using a flat representation instead of a graph
+ * of concatenated strings. This is an optimization to reduce the memory footprint of token string fragments after
+ * they are parsed.
+ * This optimization targets the V8 javascript engine and only works on Node.js.
+ * @param {string} str
+ */
+export function flatten (str: string) {
+  return Buffer.from(str).toString()
+}