[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/test_dir/BUILD.bazel

@@ -0,0 +1,14 @@
+load("//:build_defs.bzl", "flatbuffer_ts_library")
+
+flatbuffer_ts_library(
+    name = "typescript_transitive_ts_fbs",
+    srcs = ["typescript_transitive_include.fbs"],
+    visibility = ["//visibility:public"],
+)
+
+flatbuffer_ts_library(
+    name = "include_ts_fbs",
+    srcs = ["typescript_include.fbs"],
+    visibility = ["//visibility:public"],
+    deps = [":typescript_transitive_ts_fbs"],
+)

tests/test_dir/typescript_include.fbs

@@ -0,0 +1,6 @@
+include 'typescript_transitive_include.fbs';
+namespace foobar;
+
+enum class: int {
+  arguments,
+}

tests/test_dir/typescript_transitive_include.fbs

@@ -0,0 +1,5 @@
+namespace foobar;
+
+enum Abc: int {
+  a,
+}