refactor(shortcuts)!: tweak shortcuts api

[bluwy]

Description

Tighten up the shortcuts implementation a bit before Vite 5

• Made customShortcuts only an array of shortcuts. Doesn't support null | undefined array elements.
• Don't print duplicate keys for h help shortcut. E.g. if user defines the r shortcut to do something else (and not restart the server), we should only print it once.
• Don't run user-defined h shortcut as we always handle it.

Additional context

One thing I wonder is if we should allow users to override the h shortcut, but that makes the code a bit ugly and could also be tackled non-breakingly in the future.

Slightly related discussion: #8746 (comment)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[ArnaudBarre]

Personally I like the simplicity of passing null & undefined. This is something that rollup and Vite allows (with also false and promise and list)

[bluwy]

I think for server.bindCLIShortcuts specifically, its API tend to be imperative so we can be a little stricter here. The user can do .filter(Boolean) if needed. Vite configs are a little different where it's more declarative and users want a quick way to short-circuit certain plugins.