Support `configFile: false`

[YunYouJun]

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Environment

c12@latest

Reproduction

src/types

https://github.com/unjs/c12/blob/efcfbdd9549dfce6990d659fe0b34df4e6bba135/src/types.ts#L104

Describe the bug

It is mentioned in the documentation that configFile can be false, but its type is only string.

Additional context

No response

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