Fix blockquote lazy-continuation parser hang

[istvan-florio]

Summary

• Fix a parser progress bug when collecting lazy continuation lines inside blockquotes.
• Add regression coverage for the blockquote examples from Infinite loop when parsing multiline blockquotes with non-prefixed lines #46.
• Add coverage for the same lazy-continuation path inside a fenced div.

This addresses the blockquote lazy-continuation hang reported in #46.

Note: a later comment on #46 reports a footnote-adjacent hang. I reproduced that separately during local checking, and it appears to use a different parser path, so this PR does not claim to fix the footnote case.

Tests

• gleam test --target erlang
• gleam test --target javascript
• gleam format --check src test
• git diff --check

[lpil]

Thank you!