Fix npm warning by using configured default license

[mikemaccana]

Currently when a new project is made with anchor init, running anchor test will display a warning because the package.json is missing the license field:

warning package.json: No license field

This PR uses the system's configured default license (ie, what npm init -y would use) to set the license for the new project.

People can set their system's default license with npm config set init-license "<license name>". Eg

• npm config set init-license "MIT" (if you prefer MIT to be the default)
• npm config set init-license "UNLICENSED" (if you do not wish to make your code open source)

Note this is my first PR to a Rust project, so if you have any suggestions for the code please let me know!

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