[Feature Request]: Add property to `ProjectRootElement` for convenient access to `<Sdk>` elements

### Summary

Project files can contain `` elements (modeled by `ProjectSdkElement`).
For parity with other project contents, there should be a property to access only those elements (e.g. `SdkReferences`).

### Background and Motivation

There are already properties for accessing the `Sdk` attribute, all properties, all imports and all items.
However, while `Sdk` elements appear in `Children`, they have no dedicated convenience accessor of their own.

### Proposed Feature

```cs
///
/// Get a read-only collection of the SDK references.
///
public ICollection Sdks => new ReadOnlyCollection(Children.OfType());
```

(name could also be `SdkElements` for better distinction from `Sdk`; there is also `ChooseElements`)
(using `Children` and not `AllChildren` because I assume these elements are only valid at the top level)

A nice-to-have additional property (`SdkReferences`) or method (`GetSdkReferences()`) would be one that gathers all SDK references in a project, from all sources:
- `Sdk` attribute on the project
- `Sdk` elements
- `Import` elements that specify an SDK
(That would make it easy for something like Dependabot to check whether a particular project references an SDK package for which an update is available.)

### Alternative Designs

_No response_

Locate ProjectRootElement and compare its existing convenience properties with the ProjectSdkElement model and Children collection. Add a read-only accessor for the direct SDK elements, resolve the proposed naming choice, and consider the stated broader SDK-reference aggregation only if the scope is confirmed.

Written by the indexing model from the issue text.