[bazel][formatjson5] Add a BUILD.bazel

... and added basic support in bazel2gn to sync `rust_binary`.

Also took the chance to fix `formatjson5_test`, although it's not
included in CQ (why it regressed).

I mainly used formatjson5 to quickly validate a few generated
third-party Rust targets.

Change-Id: I29463e48129d058c54ef48aaec0ed3e1f60aac04
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1228644
Commit-Queue: Jay Zhuang <jayzhuang@google.com>
Reviewed-by: David Turner <digit@google.com>