Parse heredoc syntax correctly when processing completion items

[rcjsuen]

const lib = require("dockerfile-language-service");
const service = lib.DockerfileLanguageServiceFactory.createLanguageService();
console.log(service.computeCompletionItems("RUN ", { line: 0, character: 4 }).length);
console.log(service.computeCompletionItems("RUN <<eot\n", { line: 1, character: 0 }).length);

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We need to support the new heredoc syntax support added in moby/buildkit#2132. The second call is returning all the instructions as completion items when we're not actually on a new line.