Identify unused data declarations

[jrcasso]

Unused data declarations give Terraform configurations access to unnecessary information, cause code clutter, and impact runtime performance. It would be useful if linting could identify unused data declarations with syntactic highlighting.

[radeksimko]

This seems like a better fit for some kind of a linter (such as tflint), rather than a functionality built into the extension or the language server directly as I can imagine such functionality being useful outside of editors generally, e.g. in CI checks.

We still don't provide any integration with linters yet though, so I'd keep this open until we do.