Add spec for SharedClusterEnvironment.md

[Navdeep-ss]

This is an API review for the new Shared WebView2 Cluster Environment API.

It implements Approach 2 (symmetric Create + synchronous Get) from the internal design doc, the recommended proposal.

What this adds

• CreateCoreWebView2ClusterEnvironment(options, handler) - symmetric async entry point every cooperating host calls with its full desired options. First-creator-wins pins the option set for the cluster Id.
• GetCoreWebView2ClusterEnvironmentOptions(id, options) - synchronous read of the pinned options for a cluster Id, no browser spawn, for pre-flight.
• ICoreWebView2ClusterEnvironmentOptions - process-wide options plus the rendezvous Id.
• ICoreWebView2CreateClusterEnvironmentCompletedHandler - returns the shared environment, or ERROR_CLUSTER_ENVIRONMENT_OPTIONS_MISMATCH.
• .NET/WinRT projection: static CoreWebView2Environment.CreateClusterEnvironmentAsync / GetClusterEnvironmentOptions and the CoreWebView2ClusterEnvironmentOptions runtimeclass.

Notes for reviewers

• Recommended usage is "Get, then Create": read what is pinned, reuse it or offer your own, then create (establish or attach to an identical cluster).
• Storage, locking, and lifetime are summarized in the Appendix, along with the two rejected alternatives (Create/Join role split, and a getter-only shape).

Note: this PR intentionally targets the user/singhnavdeep/shared-cluster-environment-draft branch, not main.

[Navdeep-ss]

Superseded by #5633, which uses the correct review-branch naming convention (base SharedClusterEnvironment, head SharedClusterEnvironment-draft).