Licensing in source-map-support.js

[Line 286](https://github.com/evanw/node-source-map-support/blob/7b5b81eb14c9ee6c6537398262bf7dab8580621c/source-map-support.js#L286) of source-map-support.js indicates that the code that follows has been copied from V8, which is BSD-3-Clause-licensed (see e.g. their license file [here](https://chromium.googlesource.com/v8/v8/+/4.3.49/LICENSE.v8?autodive=0%2F%2F) and the original file the code seems to be copied from [here](https://chromium.googlesource.com/v8/v8/+/4.3.49/src/messages.js) - header also mentions the BSD license).

IMO the license & applicable copyright should be included in source-map-support to reflect this.