[TS] Add single-file ts codegen & bazel rule for typescript (#7161)

The headline here is adding a flatbuffer_ts_library rule for generating
typescript code in bazel. This entails some non-trivial other changes,
but ideally none are user-visible.

In particular:
* Added a --ts-flat-file flag that generates a single *_generated.ts
  file instead of separate files for each typescript type. This makes
  bazel much happier.
* Import the bazel rules_nodejs stuff needed to support building
  typescript in bazel
* Move flatbuffers.ts to index.ts because I wasn't sure how to make
  bazel comprehend the "main" attribute of the package.json. Happy
  to take another stab at figuring that out if really needed.
* Fix another couple keyword escaping spots in typescript...

package.json

@@ -9,8 +9,8 @@
     "mjs/**/*.d.ts",
     "ts/**/*.ts"
   ],
-  "main": "js/flatbuffers.js",
-  "module": "mjs/flatbuffers.js",
+  "main": "js/index.js",
+  "module": "mjs/index.js",
   "directories": {
     "doc": "docs",
     "test": "tests"
@@ -35,9 +35,11 @@
   "homepage": "https://google.github.io/flatbuffers/",
   "dependencies": {},
   "devDependencies": {
+    "@bazel/typescript": "^5.2.0",
+    "@types/node": "17.0.21",
     "@typescript-eslint/eslint-plugin": "^4.12.0",
     "@typescript-eslint/parser": "^4.12.0",
     "eslint": "^7.17.0",
-    "typescript": "^4.1.3"
+    "typescript": "^4.5.5"
   }
 }