Something to add metadata/extern flag to KModuleFields class

*(extracted from #9436)*

We have to design some non-hacky way to attach metadata to `KModuleFields` classes. There are a lot of important metas that can be applied to them, like `@:build`, `@:cppInclude`, `@:hlNative`, `@:dce`, etc.

Also, marking a module class `extern` together with `@:native`/`@:jsRequire`/etc. looks pretty useful.

I haven't really tried, but I _think_ it is currently possible to achieve this by using init macros, like `addMetadata` and `exclude`, but that's obviously a confusing, flimsy and error-prone option, hence this issue.

Here are some options that come to my mind right away:

- "magic" module declaration, like `@:build(...) class $module {}` or `@:build(...) final $module;` - it's not very pretty, but it doesn't introduce new syntax and allows for creating such modules with existing macro api with `Context.defineModule`.

- some specific module-level syntax, like `(@:build(...))`, `$ @:build(...)` or something - can look nicer, but it's new syntax, which raises questions about macro representation and tooling support.