Implements "corepack elect"

[arcanis]

See #25 and the documentation added to README.md for more details.

In short, this PR implements a proper way for projects such as create-react-app to detect which package manager to use when initializing a project, by gently asking the user only once for all installs that will occur on the machine.

[merceyz]

I'm -1 on this, I would prefer if packages check process.env.npm_config_user_agent to see which package manager was used and then do whatever they need to based on that.

[styfle]

I agree with merceyz.

For example, create-next-app documents these options for installation:

npx create-next-app@latest
yarn create next-app
pnpm create next-app

And then it uses process.env.npm_config_user_agent to determine which package manager was used.

[arcanis]

Yeah, it's an old experiment, from prior those options were implemented. I don't plan to land it anymore.