Avoid printing to stdout from Client or Ruby activation

[vinistock]

Motivation

This may be the solution for Shopify/ruby-lsp#1723.

I suspect the issue is that RVM is somehow causing something to be printed to STDOUT, which is basically left in the pipe until the LSP starts. When it finally does start, it reads STDOUT and gets the stuff that was printed, which is invalid from the LSP's perspective.

I'm not 100% sure this is the fix, but in general we should simply never use STDOUT or STDIN for anything when it comes to the client, because it may mess up the client-server communication.

Implementation

I switched all places in Client and Ruby where we were using STDOUT to get versions or perform activation. We now rely only on STDERR, which in LSPs is only used for printing and not for communication.

[andyw8]

Might another approach be to 'flush' stdout before starting ruby-lsp?

[vinistock]

I'm not sure how we'd go about doing that. It's the LSP package that owns the pipes, we don't have access to them.