Allow local functions to be extension methods in the local scope

[hrumhurum]

A code snippet that illustrates the context and need for local extension methods:

// ...

mailMessage.Subject = $"Feedback: {feedbackID} - {feedback.Product} - {feedback.Area}";
mailMessage.SubjectEncoding = Encoding.UTF8;

var sb = new StringBuilder();

const string delimeter = "==================================================================";

static StringBuilder AppendHeader(this StringBuilder sb, string header)  // <-- note 'this' keyword
{
    return sb
        .AppendLine(delimeter)
        .AppendLine(header)
        .AppendLine(delimeter);
}

// --------------------------------------------------------------------------
// DOES NOT WORK in C# 9.0 but it would be cool for this to work
// --------------------------------------------------------------------------
sb.AppendHeader($"Feedback: {feedbackID}").Append(...)

// Instead of:
sb
    .AppendLine(delimeter)
    .AppendFormat("Feedback: {0}", feedbackID)
    .AppendLine(delimeter)
    .Append(...)

[ufcpp]

This may be part of "extension methods everywhere".
related: #3994

[TheUnlocked]

#799 was also about this, though the actual conversation got slightly derailed...