Add flash functionality to session

[sdogruyol]

Summary

Adds flash message support to kemal-session, enabling temporary messages that persist only until they are read (typically used for displaying notices after redirects).

Features

• env.flash["key"] = "value" - Set a flash value
• env.flash["key"]? - Read and automatically delete (returns nil if not found)
• env.flash["key"] - Read and delete (raises KeyError if not found)

Flash messages are stored in the session with a _flash_ prefix and are automatically deleted after being read, making them ideal for one-time notifications.

Usage

Set a flash message

get "/" do |env|
  env.flash["notice"] = "Welcome!"
  env.redirect "/dashboard"
end

Read and consume the flash message

get "/dashboard" do |env|
  if notice = env.flash["notice"]?
    "<div class='alert'>#{notice}</div>"
  else
    "Dashboard"
  end
end

[sdogruyol]

//cc @mamantoha

[mamantoha]

Hi @sdogruyol . Does this duplicate the functionality of https://github.com/crystal-garage/kemal-flash?

I will check later.

[sdogruyol]

@mamantoha pretty much the same API with a simpler implementation that only supports String values

[mamantoha]

LGTM