[1.x] Back function

[adrum]

Recrated #36 due to not being based on v1:

This will be used when working on empty responses in #28

Also, much of the time, I like to be able simply redirect back to the request came from on POST requests. This allows for something like:

using InertiaCore;
using Microsoft.AspNetCore.Mvc;

namespace TestCsrf.Controllers;

public class FormController : Controller
{
    public IActionResult Index()
    {
        var props = new
        {
            Success = TempData["SuccessMessage"] != null,
            Message = TempData["SuccessMessage"]?.ToString()
        };

        return Inertia.Render("Form", props);
    }

    [HttpPost]
    public IActionResult Submit([FromForm] SubmitFormRequest request)
    {
        if (!ModelState.IsValid)
        {
            return Inertia.Back();
        }

        // Process the form data
        TempData["SuccessMessage"] = $"Form submitted successfully! Name: {request.Name}, Email: {request.Email}";

        return Inertia.Back();
    }
}

public class SubmitFormRequest
{
    public string Name { get; set; } = string.Empty;
    public string Email { get; set; } = string.Empty;
    public string Message { get; set; } = string.Empty;
}