Upgrade Syn to 2.0

[Pat-Lafon]

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I would like upgrade Inkwell's dependency of Syn to the next major release, 2.0. As other projects have updated, it's quite unfortunate to have 2 versions of Syn in my project dependency tree.

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I would like to upgrade the Syn version to 2.0 and fix any of the breaking changes. I'll open a pr but the template says I should have an issue first.

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If some projects are still on Syn 1.0, then they will have multiple versions of Syn in their dependency tree.

Syn still supports a msrv of 1.56 but if this were to increase then that might affect Inkwells minimum version. I'm not 100% sure about how/when this could occur but I'll note it here.

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One could not upgrade but with major crates like clap doing so I'm not sure why one wouldn't.

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