Introduce --redis-preconnect CLI flag

[Jarred-Sumner]

What does this PR do?

The --redis-preconnect CLI flag lets you automatically connect to Redis before your code starts to load, which can improve cold start performance.

How did you verify your code works?

Internally, this is the equivalent of calling Bun.redis.connect() before the entry points load.

[robobun]

^{Updated 7:45 AM PT - Apr 8th, 2025}

❌ @Jarred-Sumner, your commit 1f5f425 has 2 failures in Build #14653:

• test/js/web/fetch/fetch.test.ts - timeout on 🍎 14 x64
• test/js/third_party/body-parser/express-memory-leak.test.ts - 1 failing on 🍎 14 aarch64

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🧪   try this PR locally:

bunx bun-pr 18862