Fix blank line indentation for modifier conditionals

[samuel-williams-shopify]

Summary

• ignore modifier if/unless nodes when calculating structural indentation depth
• add regression coverage for a guarded describe block
• cover begin ... end while and begin ... end until, which parse as while_post/until_post and are already excluded from structural depth

Verification

• bundle exec sus (117 passed, 157 assertions)
• bundle exec rubocop (17 files, no offenses)
• checked the original async-container/test/async/container/forked.rb through the patched plugin