[labs/gen-manifest] produce events/atrributes for mixins

### Should this be an RFC?

- [X] This is not a substantial change

### Which package is this a feature request for?

Analyzer (@lit-labs/analyzer)

### Description

Hi,

I have a question/suggestion regarding custom-elements.json manifst generation.

`@lit-labs/cli gen --manifest` produces a manifest where mixins for a component class are listed and linked in the class description (`mixins` field).

If I read this [comment](https://github.com/lit/lit/issues/4529#issuecomment-1915825290) correctly, downstream tooling is supposed to resolve these references (i.e. inherited, or "mixed-in" traits). They are not flattened to the class description in the manifest. This makes sense.

However, if a mixin contributes events, properties, etc.. these should be at least listed as part of the mixin description in the manifest then. From what I have tried and from what I can tell [from the code](https://github.com/lit/lit/blob/497422780bb061bae3585579dcf842c2b411e5fb/packages/labs/gen-manifest/src/index.ts#L224), currently, mixins are (correctly) treaded as functions. I also could not find test fixtures that cover this. So currently I don't see a way to transitively extract all mixin contributions to a component class from the manifest.
In practice discovering annotations from either the mixin function, the class that is constructed within the mixin, or the returned interface could be a way to transport things like @fires annotations and properties.

### Alternatives and Workarounds

* Looking through the code, I get the impression that [there are, in principle, plans to add dedicated emission code for mixins](https://github.com/lit/lit/blob/497422780bb061bae3585579dcf842c2b411e5fb/packages/labs/gen-manifest/src/index.ts#L138). Is this understanding correct?
* If not, is there currently a way to annotate mixins so that events, attributes and properties which they contribute can be discovered in the manifest?

On a side note if many hops are required to collect all the metadata for a component in a custom-elements.json manifest it maybe does make sense to have mode where the data is flattened, for the sake of simplicity for users..

Start in packages/labs/gen-manifest/src/index.ts, especially the mixin handling around lines 138 and 224, and inspect the analyzer's existing manifest-generation tests and fixtures. Define how mixin-contributed events, attributes, and properties should appear in custom-elements.json, add coverage for the relevant mixin cases, and verify the generated manifest contains the expected metadata.

Written by the indexing model from the issue text.