[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/bit-width.ts

@@ -0,0 +1,6 @@
+export enum BitWidth {
+  WIDTH8 = 0,
+  WIDTH16 = 1,
+  WIDTH32 = 2,
+  WIDTH64 = 3,
+}