Support yarn 4.x

[mgtitimoli]

Hi there,

First of all, I would like to thank you for this great plugin you did, thanks a lot!

So to the point, Today I upgraded yarn to latest version (4.0.2) and as part of the breaking changes included in the release, there is one that is breaking semantic-relase-yarn, which is that all the native plugins are now included by default, so when it tries to install it, it breaks saying the following:

A plugin named @yarnpkg/plugin-version is already installed; possibly attempting to import a built-in plugin.