Make PDK a (limited) Ruby version manager

[jay7x]

It'd be nice to use PDK-bundled Ruby with Spotify's Ruby-LSP in VSCode. There is some support for Ruby version managers described here: https://github.com/Shopify/ruby-lsp/blob/main/vscode/VERSION_MANAGERS.md.

This way a customer can use Ruby-LSP without installing any other Ruby distribution. I guess that was one of the goals behind PDK also.

I can imagine something like this:

{
  "rubyLsp.rubyVersionManager": {
    "identifier": "custom",
  },
  "rubyLsp.customRubyCommand": "pdk activate --puppet-version=8",
}

Here is how the activation script is called: https://github.com/Shopify/ruby-lsp/blob/main/vscode/src/ruby/custom.ts#L19

`${this.customCommand()} && ruby -W0 -rjson -e '${activationScript}'`

Some other notes:

1. Just changing PATH was not enough in my case
2. Ruby-LSP will add few gems which might affect things