Enable GeneratedRegex on partial properties

[stephentoub]

Extends the attribute to apply to properties, and updates the source generator to support:

[GeneratedRegex("...")]
private static partial Regex Prop { get; }

in addition to the already supported:

[GeneratedRegex("...")]
private static partial Regex Method();

Closes #77797

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[eiriktsarpalis]

Presumably this would require conceptual documentation updates?

[stephentoub]

Presumably this would require conceptual documentation updates?

Yes, we'll want to augment them to highlight the additional feature.