feat(redis): support ioredis v6

[mattstrayer]

Closes #18

Problem

@verrou/core limits the ioredis peer range to ^5.3.2. ioredis 6.0.0 is now latest. When another dependency needs v6, pnpm installs a private second copy of v5 for verrou. The two copies have different type identities, so passing the app's v6 client to redisStore({ connection }) fails typecheck.

Changes

1. Widen the peer range to ^5.3.2 || ^6.0.0.
2. Type the connection option as a structural RedisConnection interface that lists the five commands the driver calls (set, setnx, eval, del, get). It replaces the import of the ioredis Redis class. A v5 client, a v6 client, a Redis<"resp3"> client and a Cluster instance all satisfy it, and the option no longer depends on the class shape.
3. Add a type test that asserts Redis and Cluster match RedisConnection.
4. Docs: note that v5 and v6 both work and that v6 needs Node.js 20 or newer.

The devDependency stays at ^5.8.2, so the lockfile is untouched.

Verification

With ioredis 5.8.2 installed:

• tsc --noEmit: no errors
• eslint .: no errors
• pnpm build: passes
• tests/drivers/redis.spec.ts against Redis 8: 27 tests pass

With ioredis 6.0.0 installed in place of 5.8.2:

• tsc --noEmit: no errors
• Same 27 tests pass
• Redis<'resp3'> is assignable to RedisConnection