Add support for partial indices

Postgres supports partial indices by passing a where clause to the CREATE INDEX statement, as such:

```sql
CREATE UNIQUE INDEX ON "users" ("email") WHERE NOT "is_deleted";
```

Useful in some cases with soft deletions.

No files or tests are named. Start by locating the entry point that generates PostgreSQL CREATE INDEX statements and inspect how index options are represented. Done means partial indices can emit a WHERE clause, including the soft-deletion example shown in the issue.

Written by the indexing model from the issue text.