Ability to force an interface to be considered as a struct

## :rocket: Feature Request

### Affected Languages

- [X] `TypeScript` or `Javascript`
- [ ] `Python`
- [ ] `Java`
- [ ] .NET (`C#`, `F#`, ...)

### General Information

Sometimes, a struct's "natural" name could start with a capital letter `i` followed by another capital letter. This in fact happens in several instances in the `aws-cdk` (`IAMThis`, `IPThat`, ...). This is particularly challenging to work around in cases where code is generated, as it may not always be obvious how to case-adjust those names to avoid the `I[A-Z]` prefix.

Unfortunately, `jsii` will consider any interface named with an `I[A-Z]` prefix as a behavioral interface, which significantly degrades the user experience around those types. This proposal is to add an optional `@struct` TSDoc annotation that will "force" a particular interface to be interpreted as a struct, regardless of it's name (it will still be required to be a valid struct, meaning it cannot declare any method and must have only `readonly` properties).

This would be the most elegant solution to aws/aws-cdk#11501.

The issue names no files, tests, or entry points. Start by locating jsii's interface classification and TSDoc annotation handling, then trace how struct validity is checked; done means a valid readonly, method-free interface with an I[A-Z] name can opt into struct treatment through @struct, with coverage for the annotation and invalid cases.

Written by the indexing model from the issue text.