separate LicenseVersion to an Library for integrating easily with NFT projects

[Stevewu1234]

we've utilized the a16z License contract in our ETHGlobal online hackathon NFT projects: LeapStart, And we found that it is hard to integrate with the enum LicenseVersion when the NFT contract is created by a factory contract and the LicenseVersion will be given from other contracts. Therefore, we'd like to separate the enum LicenseVersion into a Library contract for integrating easily with a more complicated contract system.