[w3c/selection-api] Add `selectionRoot` option to getComposedRanges parameter options (Issue #180)

To simplify selection for components that only wish to handle selections within a sub-tree, we would like to add the optional selectionRoot parameter. If provided, the returned StaticRange from getComposedRanges() will only contain endpoints that are inclusive descendants of this selectionRoot.

Proposal:
```
sequence<StaticRange> getComposedRanges(optional GetComposedRangesOptions options = {});

dictionary GetComposedRangesOptions {
  sequence<ShadowRoot> shadowRoots = [];
  selectionRoot = null; // new
};
```

We would also need to redefine how the rescoping works for selectionRoot:
* If selection range's start node is outside/above the selectionRoot, it is rescoped to be the first position within selectionRoot.
* If selection range's end node is outside/above the selectionRoot, it is rescoped to be the last position within selectionRoot.

## Resources
This parameter was first suggested in the [explainer](https://github.com/mfreed7/shadow-dom-selection?tab=readme-ov-file#re-scoping) and had positive feedback. Please see the example in the explainer as it illustrates this parameter well.

Related issue: https://github.com/w3c/selection-api/issues/176


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