Support multiple directives when calculating symbols

[rcjsuen]

let dockerfileLanguageService = require("dockerfile-language-service");
let service = dockerfileLanguageService.DockerfileLanguageServiceFactory.createLanguageService();
let symbols = service.computeSymbols(
  {
    "uri": "file:///tmp/Dockerfile"
  },
  "#escape=`\n#syntax=docker/dockerfile"
);
console.log(symbols.length);

1

2 should be printed as there are two directives.