Initializer alternative to fluent AcceptLegalFilePathsOnly

[KalleOlaviNiemitalo]

I tried porting an existing app from System.CommandLine 2.0.0-beta4.22272.1 (commit 209b724) to 2.0.0-beta4.23073.1 (commit 6524142). Because of #1994, it is no longer possible to initialize a static field like this:

        private static readonly Option<FileInfo> OutputZipFileOption
            = new Option<FileInfo>(
                aliases: new[] { "-o", "--output" })
            {
                IsRequired = true,
            }
            .LegalFilePathsOnly();

Instead, one can do any of the following:

• Add a private static Option<FileInfo> CreateOutputZipFileOption method and call that in the initializer.
• Explicitly define a static constructor that initializes the fields. This loses the beforefieldinit flag but that seems unlikely to matter for performance because the initialization would have been done during startup anyway.
• Make the fields not static, and initialize them in an instance constructor.
• Replace the fields with local variables, and reference them from a lambda or a local function so the C# compiler lowers them to fields again.
• Wait for [Proposal]: Directly invoked anonymous functions csharplang#4748.

Could the library instead allow this syntax:

        private static readonly Option<FileInfo> OutputZipFileOption
            = new Option<FileInfo>(
                aliases: new[] { "-o", "--output" })
            {
                IsRequired = true,
                Validators = { OptionValidation.LegalFilePath },
            };

That would be more convenient to use, but more difficult to discover. The documentation of the Validators property could reference it, though. It might even have been possible to guide the Intellisense feature of Visual Studio to it by using the completionlist element in the XML documentation comments of ValidateSymbolResult<T>, if that type had not been removed in #1944 / #1946.

[KalleOlaviNiemitalo]

Comment moved to #1809 (comment).