Use valid SPDX licence in package.json

[silverwind]

The current specifier is not a valid SPDX expression, see https://docs.npmjs.com/cli/v7/configuring-npm/package-json#license for options on how to declare dual-licensing. Given that the README states that the license is mirroring jQuery, I opted to just copy their value from https://github.com/jquery/jquery/blob/482f846203e82b1c2620f580e483bf41d11f9f49/package.json#L25.