feat(rosetta): hoist declare statements to top-level of document

[BenChaimberg]

declare statements allow snippet writers to create a mock variable of a certain type that can be used within a snippet without having to provide a concrete value of that type. This is useful when referencing a type from another module without needing to create an instance of that type, which may be complex. Example:

declare const lambdaFunction: lambda.Function;
const myResource = ...
myResource.grantRead(lambdaFunction);

declare statements can only exist at the top level of a document, meaning snippets with fixtures may not be able to define a declare statement, since the snippet may be embedded into the fixture within a class, function, etc. This feature "hoists" declare statements to the top level of the document, similarly to how import statements are hoisted currently.

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[eladb]

Cool stuff

[RomainMuller]

👌🏻 Thank you so much!

[mergify]

Thank you for contributing! ❤️ I will now look into making sure the PR is up-to-date, then proceed to try and merge it!