Add ShowMessageRequest API to psEditor

Right now we have the send message notifications hooked up, but there's also a request that looks like this:

It would be cool to have that hooked up in a generally accessible way. Right now you can do this:

using namespace OmniSharp.Extensions.LanguageServer.Protocol.Models
using namespace OmniSharp.Extensions.LanguageServer.Protocol.Server
using namespace OmniSharp.Extensions.LanguageServer.Server


# Type name is fully qualified because there's an ILanguageServer
# in both the Protocol and the Server namespaces.
$server = $psEditor.Components.GetService(
    [OmniSharp.Extensions.LanguageServer.Protocol.Server.ILanguageServer])

$task = [ShowMessageRequestExtensions]::Request(
    $server.Window,
    [ShowMessageRequestParams]@{
        Type = [MessageType]::Error
        Message = 'Question that is relevant to the options?'
        Actions = (
            [MessageActionItem]@{ Title = 'Option 1' },
            [MessageActionItem]@{ Title = 'Option 2' })
    })

while (-not $task.AsyncWaitHandle.WaitOne(200)) { }

# This will hold the chosen MessageActionItem
$result = $task.GetAwaiter().GetResult()

Something like this would be great though:

public partial class EditorWindow
{
    public string SendMessage(
        MessageType messageType,
        string caption,
        params string[] options);
}

Start with the psEditor EditorWindow API and trace the existing ShowMessageRequestExtensions call through ILanguageServer and its Window service. Compare the requested SendMessage signature with the current notification support and determine how the selected MessageActionItem should be exposed. Done means a generally accessible EditorWindow API can present the options and return the user's choice.