Add shapes-only codegen

Would like to have the option to generate only data shapes similar to the feature in smithy-java described in https://github.com/smithy-lang/smithy/issues/2514 ([example](https://github.com/smithy-lang/smithy-java/tree/0.0.1/examples/standalone-types)) and in the works for smithy-python per https://github.com/smithy-lang/smithy-python/pull/373.

Start by reading the linked smithy-java standalone-types example and the referenced smithy-python pull request to understand the intended shapes-only behavior. Then locate the corresponding smithy-go code-generation entry points; done means an option can generate data shapes without the usual additional generated code.

Written by the indexing model from the issue text.