Generate license.json based on GN dependencies

[theodab]

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We can use gn desc to determine what a target depends on. Using this, we can determine what third-party things a target depends on. The app generates a json file containing the licenses of third-party dependencies, but this file is based on a static list of dependencies. We should use gn desc to determine the third-party dependencies automatically. This will ensure we don't forget to update it, will make adding this to the Mac framework easier, and allow us to use this on Linux too.