Usage of IIncrementalGenerator and InterceptsLocationAttribute to vastly improve performance

[just-ero]

The use of source generation via IIncrementalGenerator and the C#12 preview feature "interceptors" may be able to improve Spectre.Console's performance by a non-trivial amount.

Interceptors allow for compile-time rerouting of method calls to different code. This means that, for example, a call to AnsiConsole.MarkupLine could be intercepted and instead a call to Console.WriteLine with the parsed markup can be inserted instead. This requires source generation of each parsed call to AnsiConsole.MarkupLine.

As an example, take a simple Program.cs like this;

using Spectre.Console;

AnsiConsole.MarkupLine("[Green]Hello, world![/])");

A source generator would pick up this call to AnsiConsole.MarkupLine, perform the markup parsing, and would generate an interceptor such as this;

// generated code
static partial class AnsiConsoleMarkupLineInterceptors
{
  [System.Runtime.CompilerServices.InterceptsLocation("Program.cs", line: 3, character: 13)]
  public static void ProgramCs_3_13(string value)
  {
    Console.WriteLine("\x1b[38;2;0;255;0mHello, world!\x1b[0m");
  }
}

This would allow for the parsing of markup commands to happen at compile time and would speed up execution.