[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.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
-    "target": "es5",
+    "target": "ES5",
     "module": "commonjs",
-    "lib": ["es2015"],
+    "lib": ["ES2015", "ES2020.BigInt", "DOM"],
     "declaration": true,
     "outDir": "./js",
     "strict": true,
@@ -11,6 +11,6 @@
     "forceConsistentCasingInFileNames": true
   },
   "include": [
-    "ts/*.ts"
+    "ts/**/*.ts"
   ]
 }