[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...

tests/tsconfig.json

@@ -15,6 +15,7 @@
   "include": [
     "monster_test.ts",
     "typescript_keywords.ts",
+    "typescript_keywords_generated.ts",
     "my-game/**/*.ts",
     "typescript/**/*.ts",
     "optional_scalars/**/*.ts",