[stylelint-plugin] feat: Improve copyright comment detection

[kmark]

Adds support for comments like this:

// Copyright 2016 Palantir Technologies, Inc. All rights reserved.
<would add the import here before>
// Licensed under the Apache License, Version 2.0.
<adds the import here now>

Was not able to add a test for the // case since postcss doesn’t recognize these as comments by default.

[blueprint-bot]

Add a test

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