feat: adds --output to flatten

[s-di-cola]

• Because this PR includes a new feature, the change was previously discussed on an Issue or with someone from the team.

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Implements --output parameter for flatten task as requested in #3430. This allows users to specify an output file path for the flattened contract, improving usability especially on Windows systems where shell redirection may not work consistently.

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Name	Status	Preview	Comments	Updated (UTC)
hardhat	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 22, 2024 4:18pm

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Latest commit: 31c0a94

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[fvictorio]

This looks good, but I'll let someone from the Hardhat team actually review it in depth and approve it. This also needs a changeset, of course.