Display add-on version in the displayAddons command

[st0012]

Motivation

Since add-ons now can specify their versions, we can also display them in VS Code.

Implementation

• Add version to rubyLsp/workspace/addons's response
• Add a nullable version attribute to the Addon extension class as older versions of ruby-lsp don't return it
• Add version information to displayAddon command's output:

Automated Tests

Updated an existing case for it.

Manual Tests

1. Run Ruby LSP extension with this branch
2. Use the latest ruby-lsp-rails as the development host
3. Update ruby-lsp-rails's Gemfile to use this branch of ruby-lsp
   • gem "ruby-lsp", github: "Shopify/ruby-lsp", branch: "display-add-on-version"
   • Run bundle update
4. After Ruby LSP is initialized, run Ruby LSP: Display addons command in VS Code's command palette

[vinistock]

This may be a bit premature. If add-ons haven't upgrade to the latest API yet, the moment the extension is released it's going to break. Why do we need to add this immediately? Can it wait a bit until people had the chance to upgrade?