Add protobuf code comments as Elixir module documentation

[btkostner]

This PR takes code comments from the protobuf files and add them as @moduledoc blocks. Some of the code and all of the magic numbers are taken from protokit golang library. This is the same parser logic used by https://github.com/pseudomuto/protoc-gen-doc which is pretty popular.

Example generated code:

defmodule My.Test.Days do
  @moduledoc """
  This enum represents days of the week.
  """

  use Protobuf, enum: true, protoc_gen_elixir_version: "0.12.0", syntax: :proto2

  field :MONDAY, 1
  field :TUESDAY, 2
  field :LUNDI, 1
end

Todos:

• Make the code cleaner. The recursive index based path for comments is pretty annoying to deal with.
• Get documentation for messages working
• Get documentation for services working
• Documentation for fields? (This does nothing, but would be nice for future improvements)
• Tests

[btkostner]

Alright, this should be a good starting point. It generates the code comments as expected. Code is pretty cleaned up. I think adding some work for documenting individual fields will be the next step, but we should be good here so far.

Fixes #266

[Nezteb]

@btkostner Regarding your comment:

Documentation for fields? (This does nothing, but would be nice for future improvements)

Could you elaborate on the "this does nothing" part? 😄

[btkostner]

@btkostner Regarding your comment:

Documentation for fields? (This does nothing, but would be nice for future improvements)

Could you elaborate on the "this does nothing" part? 😄

Two ways to document the fields. Add a doc option for fields which I think would be preferred. Otherwise, just create a @typedoc block with each field documented. I decided not to implement the field documentation until the team decides on the way forward.

[Nezteb]

until the team decides on the way forward

@whatyouhide @wingyplus Y'all show up often in the git history of the files Blake modified, so I feel like you are the most qualified to chime in. Do either of you have a preference on field docs?

I'd add you as PR reviewers but I don't have permissions. 😅

[v0idpwn]

@whatyouhide @ericmj I'd really like to merge this. The only potential blocker I see are warnings of this type for Elixir 1.12 / OTP 24:

The current heredoc line is indented too little
  nofile:6

warning: outdented heredoc line. The contents inside the heredoc should be indented at the same level as the closing """. The following is forbidden:

    def text do
      """
    contents
      """
    end

Instead make sure the contents are indented as much as the heredoc closing:

    def text do
      """
      contents
      """
    end

I'm not sure why this happens yet, trying to understand. They happen during generation, but not on actual generated code.

Edit: found it, fixed. Would you please take a brief look before I merge?

[v0idpwn]

Well, I'm merging :)
If there's a post-merge review, I can fix any comments.

Thank you, @btkostner!
Over the next weeks, I'll try to get to your Google protos PR too.