Refine README

[Copilot]

The README had a duplicate ## Features heading and lacked structure in key areas, making it harder to scan.

Changes

• Fixed duplicate heading — second ## Features block (Enum/Unicode/Fluent examples) renamed to ## Additional features
• Split Installation from Quick start — quick-start code samples moved to a dedicated ## Quick start section; Installation is now install-commands only
• Added ## Validators section — reference table for all four built-in validators (Required, MinLength, MaxLength, RegularExpression), which were used in examples but undocumented
• Added syntax highlighting to bare code fences: Install-Package → powershell, dotnet add/dotnet run → sh

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/graphql
  • Triggering command: /usr/bin/gh gh issue list --limit 20 (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

README を洗練させてください

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