Parse heredoc syntax for ADD and COPY when calculating completion items

[rcjsuen]

There is still the file2 heredoc to process so we should not be prompting any completion items. We should include ADD and COPY in our heredoc support.

ADD a.txt <<file b.txt <<file2 /destination/
abc
file

COPY a.txt <<file b.txt <<file2 /destination/
abc
file

const lib = require("dockerfile-language-service");
const service = lib.DockerfileLanguageServiceFactory.createLanguageService();
console.log(service.computeCompletionItems("ADD a.txt <<file b.txt <<file2 /destination/\nabc\nfile\n", { line: 3, character: 0 }).length);
console.log(service.computeCompletionItems("COPY a.txt <<file b.txt <<file2 /destination/\nabc\nfile\n", { line: 3, character: 0 }).length);

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