Implement --version flag for cli

[ntkme]

Closes #31.

Although the embedded protocol can provide this information via VersionRequest, there is a chicken egg problem:

• Without making a VersionRequest, host doesn't know the correct protocol_version to use to talk to embedded.
• Without knowing the correct protocol version to use, a VersionRequest may fail. This is very unlikely, but could happen in theory.

Therefore it is helpful to implement a --version flag in the cli interface that prints the same information as VersionReponse:

$ ./build/dart-sass-embedded --version
{
  "protocol_version": "1.0.0-beta.12",
  "compiler_version": "1.0.0-beta.9",
  "implementation_version": "1.38.0",
  "implementation_name": "Dart Sass"
}

This would allow the host implementation to check and handle compatibility issues better. Host could also download the proper version of proto file and recompile as needed, etc.e

[nex3]

I like the idea here, but there are a couple things I'd like to see:

• Rather than coming up with a custom JSON format, emit the JSON encoding of the VersionResponse proto (with id set to 0).
• Describe the behavior of this flag in the readme.

[ntkme]

Updated to print JSON representation of versionResponse.

$ ./build/dart-sass-embedded --version
{
  "protocolVersion": "1.0.0-beta.12",
  "compilerVersion": "1.0.0-beta.9",
  "implementationVersion": "1.38.0",
  "implementationName": "Dart Sass",
  "id": 0
}

[nex3]

Thanks for your contribution! I'll merge this once I update the compiler to know about Value.HwbColor.