Add definition support for heredocs

[rcjsuen]

We should be able to go from the delimiter back to the declaration.

FROM alpine
RUN echo <<file
abc
file

const lib = require("dockerfile-language-service");
const service = lib.DockerfileLanguageServiceFactory.createLanguageService();
const definition = service.computeDefinition({ uri: "uri://Dockerfile" }, "FROM alpine\nRUN echo <<file\nabc\nfile", { line: 3, character: 2 });
console.log(definition);

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