Add a .NET API to add native completers

Summary of the new feature/enhancement

Currently there are ways to register a scriptblock as a completer with Register-ArgumentCompleter, and on cmdlets it’s possible to use the [ArgumentCompleter()] attribute to specify an IArgumentCompleter for a parameter. But there’s no way to specify a completer for an ordinary command that I know of that doesn’t involve executing a scriptblock.

By this I mean, you can use Register-ArgumentCompleter or you can use override TabCompletion2, but I’d really like a nicer hook for:

• General completions
• Command completions
• Native command completions, especially

[SeeminglyScience]

I’d love to see more ways of extending completion! One of the biggest things that’s always stopped me from working on anything to extend general completion is that it’s all or nothing. If you override TabExpansion2, then anyone else trying to do the same will completely override you. I’d love to see something like an interface that can be registered in the engine to incrementally augment completion. Something like:

public interface ICompletionExtension
{
    IEnumerable<CompletionResult> CompleteComment(CompletionContext context);

    IEnumerable<CompletionResult> CompleteMember(CompletionContext context);

    IEnumerable<CompletionResult> CompleteCommandParameter(CompletionContext context);

    IEnumerable<CompletionResult> CompleteCommandArgument(CompletionContext context);

    // Other cherry picked methods from CompletionCompleters/CompletionAnalysis here.
}

public class CompletionEngine
{
    public void RegisterCompletionExtension(ICompletionExtension extension);

    public void UnregisterCompletionExtension(ICompletionExtension extension);

    // Mirroring CommandCompletion.CompleteInput
    public CommandCompletion CompleteInput(
        Ast ast,
        Token[] tokens,
        IScriptPosition positionOfCursor,
        Hashtable options);
}

public partial class EngineIntrinsics
{
    public CompletionEngine Completion { get; }
}

[iSazonov]

So we could discuss follow signatures:

Register-ArgumentCompleter -Command <string[]>  -CompleterType [Custom type with ICommandCompleter]
Register-ArgumentCompleter -ParameterName <string> [-Command <string[]>]  -CompleterType [Custom type with ICommandCompleter]

public void RegisterArgumentCompleter(string commandName, ICommandCompleter completer);
public void RegisterArgumentCompleter(string[] commandName, ICommandCompleter completer);
public void RegisterArgumentCompleter(IEnumerable<string> commandName, ICommandCompleter completer);

public void RegisterArgumentCompleter(string? commandName, string parameterName, IArgumentCompleter completer);
public void RegisterArgumentCompleter((string? commandName, string parameterName) commandparameterPair, IArgumentCompleter completer);
public void RegisterArgumentCompleter(IEnumerable<string?, string> commandparameterPair, IArgumentCompleter completer);