Add Schema string field to Migrator. When set, Applied() constructs a
schema-qualified table name via pgx.Identifier.Sanitize() rather than
the bare "_migrations". New() signature is unchanged.
Usage:
runner := pgmigrate.New(conn)
runner.Schema = "authz"
pgx/v5's Conn.Query is lazy — when the queried table doesn't exist,
the 42P01 error doesn't surface at Query() time, it surfaces at
rows.Err() after the iteration loop. The original code only checked
for 42P01 at the Query() site, so first-run migrations against an
empty database failed with:
reading rows: ERROR: relation "_migrations" does not exist (SQLSTATE 42P01)
Apply the typed-error check at both sites via a shared helper.
Migrations run sequentially on a single connection — a pool adds
unnecessary complexity and forces callers to create one. This also
drops the puddle/v2 and x/sync transitive dependencies.
