Auto import use modules when snippet is triggered

[patriciobcs]

Currently, an snippet only add itself to the codebase but its dependencies are not added automatically.

VSCode Snippets doesn't allow the declaration of modules/imports in the snippets.json. However, Rust Analyzer allows the declaration of snippets overwritting the propiety rust-analyzer.completion.snippets.custom.

To resolve this issue, the snippet-lang should include snippet-requires-start/end tags and the compiler should be able to convert them to the Rust Analyzer format. The location of the snippets will be the inside the contributes of the extension's package.json.