Centralize and clean up diagnostic IDs

[roji]

Centralizes all diagnostic IDs in a new public EFDiagnostics class:

public static class EFDiagnostics
{
    public const string InternalUsage = "EF1001";
    public const string InterpolatedStringUsageInRawQueries = "EF1002";
    public const string SuppressUninitializedDbSetRule = "EFSPR1001";

    // Diagnostics for [Experimental]
    public const string ExperimentalApi = "EF9001";
    public const string ProviderExperimentalApi = "EF9002";
    public const string PrecompiledQueryExperimental = "EF9100";
}

So EF9xxx is for all the experimental stuff, EF9001 is for user-facing experimental APIs which don't have a dedicated ID, EF9002 is for provider-facing ones, and EF9100 and above is for specific experimental feature IDs (such as precompiled queries).