[JS/TS] Rewrite flexbuffers JS to TS (#6148)

* Partial TS rewrite

* Completed port but bugs remain

* Expose builder function

* Break out and fix stack-value and formatting

ts/flexbuffers/flexbuffers-util.ts

@@ -0,0 +1,9 @@
+export function fromUTF8Array(data: BufferSource): string {
+  const decoder = new TextDecoder();
+  return decoder.decode(data);
+}
+
+export function toUTF8Array(str: string) : Uint8Array {
+  const encoder = new TextEncoder();
+  return encoder.encode(str);
+}