[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

tsconfig.mjs.json

@@ -2,7 +2,7 @@
   "compilerOptions": {
     "target": "ES2017",
     "module": "ES2015",
-    "lib": ["ES2017"],
+    "lib": ["ES2017", "ES2020.BigInt", "DOM"],
     "declaration": true,
     "outDir": "./mjs",
     "strict": true,
@@ -11,6 +11,6 @@
     "forceConsistentCasingInFileNames": true
   },
   "include": [
-    "ts/*.ts"
+    "ts/**/*.ts"
   ]
 }