Implement a nullable annotation for fields and messages

[rockwotj]

Closes #24

[ribrdb]

I can't load these generated rules into the firestore emulator:

    Error: 3 INVALID_ARGUMENT: Error compiling rules:
    L14:101 Unexpected 'null'.

Should that be == null instead of is null? I had a couple other observations after using this for a while:

• I think what I really want is to be able to set this by type. In particular I'd like a file option to make dates and references nullable.
• I'm working on an ORM to generate typesafe code in multiple languages (go, typescript, java) that enforces these schemas. As part of that I've been working on bazel macros for working with firestore rules and testing them using the firestore emulator. I was planning to release them with my ORM, but I'm wondering now if you'd want to put them here. I have:
  • firestore_rules_library and firestore_rules_binary for combining multiple files into a complete .rules file
  • firestore_rules_proto_library to generate the .rules file using this generator and put it into a firestore_rules_library
  • an example of starting and using the firestore emulator from a jasmine_node_test

[rockwotj]

It should be ==, thanks for catching

I'd welcome contributions for the per type config! It shouldn't be too bad.

RE: bazel rules, I think those specific rules would be great to have in this repo and I'd welcome those contributions!

I think as long these macros/rules are primatives and not specialized to the ORM, this repo is a good place for them.