chore: replace document plain text with paragraphs

[Horusiath]

This PR replaces Document::to_plain_text with Document::paragraphs returning blocks as plain text paragraph list, which is more versatile option:

1. We can group paragraphs together: for the purpose of future text chunking for the appflowy-cloud document indexer.
2. new_line_each_paragraph parameter is not necessary: just use Document::paragraphs().join("\n") if necessary.
3. empty_space_each_delta parameter is also not necessary: current logic trims the preceding and leading whitespaces but only adds extra spaces only if we actually trimmed the content.