Replace the interim monospace table fallback with Slack's native `table`
block (rows of rich_text cells). Addresses the core ask in #18918.
- _table_block(): builds type:"table" with rich_text cells, so inline
formatting (bold, links, code) renders inside cells.
- Column alignment parsed from the markdown separator row (:---, :-:, --:)
into column_settings (left = default/null-skip, center/right emitted).
- Escaped pipes (\\|) are not treated as column separators.
- Respects Slack's table limits (100 rows / 20 cols / 10k aggregate chars);
oversized or unparseable tables gracefully fall back to aligned monospace
(rich_text_preformatted), so a big table never breaks the message.
Docs (EN + zh-Hans) updated to describe native tables + the fallback.
Tests: native table shape, alignment->column_settings, inline-formatted
cells, oversized/too-wide monospace fallback, escaped-pipe cell. Prove-
failed against a stubbed _table_block (native-table tests fail, fallback
tests stay green). All existing Slack tests still pass.
7c7b489813