feat: support multiple RSpec directories

[sablecode]

Summary

Adds support for configuring rubyTestExplorer.rspecDirectory as either a single directory or an array of directories.

Changes

• allow rubyTestExplorer.rspecDirectory to be a string or string array in the extension config schema
• include all configured RSpec directories when building the rspec --pattern glob
• update test tree path handling so files from multiple spec roots are grouped correctly
• update the save watcher so edits in any configured RSpec directory trigger reloads
• document array usage in the README

Testing

• npm install --ignore-scripts
• npm run build

Closes #50