DEV: Add ruby_lsp gem to development (#20517)

In order to use the ruby-lsp vscode extension, the ruby_lsp gem needs to
be added to the project's Gemfile. That may soon change with
https://github.com/Shopify/vscode-ruby-lsp/pull/419 but this will do for
now.

Gemfile

@@ -181,6 +181,7 @@ group :development do
   gem "binding_of_caller"
   gem "yaml-lint"
   gem "yard"
+  gem "ruby-lsp", require: false
 end
 
 if ENV["ALLOW_DEV_POPULATE"] == "1"