Add optional structure top of code blocks

[stephenh]

I.e. to enable plugins/approaches like:

function myPlugin(output: TsProtoOutput) {
  const file = output.getFile("./simple.ts");
  const foo = file.getClass("FooRpcService");
  const getFoo = foo.getMethod("getFoo");
  foo.setMethod("getFoo", code`
    ...my customization...
    ${getFoo}
    ...my customization...
  `);
  return output;
}

Not sure if it's best to have full-fledged File / Class / Function / Method types, or if something as simple as object hashes would be good enough.