[Feature]: Expose `DismissableLayer` and its layer-stack context as a public API

[kolirt]

Describe the feature

Currently DismissableLayer (and its context reactive store with layersRoot, layersWithOutsidePointerEventsDisabled, branches) is shipped in dist/DismissableLayer/, but is not declared in package.json#exports and not re-exported from the public entry point. This makes it impossible for third-party libraries to integrate with the dismissable-layer stack that reka-ui maintains internally.

I would like to propose making DismissableLayer (the component) part of the public API, optionally together with a small composable / accessor that lets external code participate in the same layer stack.

Additional information

• I intend to submit a PR for this feature.
• I have already implemented and/or tested this feature.

[zernonia]

Thanks for the suggestion @kolirt ! However I would like to keep it private for now, as I have plan to further improve the approach to DismissableLayer, and want to keep the breaking change to minimal