Add manual Ruby configuration

[vinistock]

Motivation

Closes #1942

This is the final step to match the functionality in Shopify/vscode-ruby-lsp#923 and allow us to stabilize our prerelease version.

This PR allows users to manually select a Ruby installation both for specific workspaces or a global fallback in case they are not using a version manager.

Implementation

1. If something fails during activation, we offer to either retry or manually configure Ruby
2. In the manual configuration dialog, we allow setting either workspace or global Ruby path
3. If a workspace path is set, we use that to launch Ruby
4. If a global path is set, we use that as a fallback if regular activation fails