Support custom RSpec debug commands

[tinyopsstudio]

Summary

• Add an optional rubyTestExplorer.rspecDebugCommand setting for Spring/rdbg RSpec debug runs.
• Add rubyTestExplorer.debuggerConfig overrides so the attach session can use alternative debugger types such as rdbg.
• Preserve the extension process env for RSpec dry-runs and recognize rdbg readiness messages before attaching.

Fixes #110.

Testing

• npm run build
• npm run test:rspec (5 passing; required temporarily linking Homebrew Ruby because the fixture lockfile requires Bundler 2.7.2)