Add COMMENT ON support for database objects

[glommer]

Implement the COMMENT ON statement to attach human-readable descriptions to tables, columns, indexes, views, and types. Comments are stored in a lazily-created __turso_internal_comments table. IS NULL removes a previously set comment.

Add PRAGMA comment_list with optional object_name filtering. .schema now formats CREATE TABLE as multi-line (one column per line) and injects inline -- comments when present. .dump emits COMMENT ON statements instead of raw internal table DDL so comments survive dump/restore.

Move cmdlineshell.sqltest to turso-tests/ because .schema now produces multi-line CREATE TABLE output which diverges from sqlite3 single-line format. Move comment_on.sqltest to turso-tests/ since COMMENT ON is a Turso extension not present in SQLite.

[turso-bot]

Please review @LeMikaelF

[codspeed-hq]

Merging this PR will not alter performance

✅ 311 untouched benchmarks
⏩ 105 skipped benchmarks¹

---

_{Comparing glommer:comments (3076915) with main (8425cba)}

Footnotes

1. 105 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩