Add library section to generated Cargo.toml

[dmitrii-ubskii]

What is the goal of this PR?

We add [lib] section to the crate manifest, specifying the library root. This allows us to have non-lib.rs root to our library in the assembled crate.

[typedb-bot]

PR Review Checklist

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Trivial Change

• This change is trivial and does not require a code or architecture review.

Code

• Packages, classes, and methods have a single domain of responsibility.
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• The code is canonical and the minimum required to achieve the goal.
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• Logic and naming has clear narrative that communicates the accurate intent and responsibility of each module (e.g. method, class, etc.).
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Architecture

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