feat: allow passing down "null" to disable server watcher

[sheremet-va]

Description

Fixes #14189

Providing null in server.watch config option (similar to build.watch) now disables the watcher altogether.

Additional context

This helps with watch leaks in Vitest.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[AriPerkkio]

I built some test cases earlier today. Check the playground/watch: AriPerkkio@16162d6

[bluwy]

+1 for this feature. This is also useful for Astro as it spins up a one-off server to load the config, and watching would be disabled.

Have one nit below.

[bluwy]

IIRC in the last meeting we're ok with this as it doesn't change how plugins interact with the watcher (much).

[bluwy]

The watcher test is a little unstable so I updated it. It's more naive but I think probably good enough.