[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...

BUILD.bazel

@@ -8,6 +8,7 @@ package(
 
 exports_files([
     "LICENSE",
+    "tsconfig.json",
 ])
 
 config_setting(
@@ -79,6 +80,7 @@ cc_library(
 # Public flatc compiler.
 cc_binary(
     name = "flatc",
+    data = ["//reflection:reflection_fbs_schema"],
     deps = [
         "//src:flatc",
     ],