[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/BUILD.bazel

@@ -1,5 +1,5 @@
 load("@rules_cc//cc:defs.bzl", "cc_test")
-load("//:build_defs.bzl", "flatbuffer_cc_library")
+load("//:build_defs.bzl", "flatbuffer_cc_library", "flatbuffer_ts_library")
 
 package(default_visibility = ["//visibility:private"])
 
@@ -190,3 +190,12 @@ flatbuffer_cc_library(
         "--cpp-ptr-type flatbuffers::unique_ptr",
     ],
 )
+
+flatbuffer_ts_library(
+    name = "typescript_ts_fbs",
+    srcs = ["typescript_keywords.fbs"],
+    deps = [
+        "//tests/test_dir:include_ts_fbs",
+        "//tests/test_dir:typescript_transitive_ts_fbs",
+    ],
+)