Add CSharp Console to PROJECTS.rst

[niqibiao]

Hi! Could you add CSharp Console to the Shells section of PROJECTS.rst?

CSharp Console is an open-source interactive C# REPL for Unity. Its Python terminal client uses prompt_toolkit and connects to a Unity Editor or Player, allowing users to execute C# and inspect live project state. Compilation and semantic completion are provided by Roslyn on the Unity side.

The client uses prompt_toolkit for its terminal interface, custom completion providers with fuzzy matching, key bindings, and theme styling.

• Project and demo: https://github.com/niqibiao/unity-csharpconsole#how-it-looks
• Python REPL source: https://github.com/niqibiao/unity-csharpconsole/tree/main/Editor/ExternalTool~/console-client
• Dependency declaration: https://github.com/niqibiao/unity-csharpconsole/blob/main/Editor/ExternalTool~/console-client/requirements-repl.txt
• License: Apache-2.0

Suggested entry:

- `CSharp Console <https://github.com/niqibiao/unity-csharpconsole>`_: An interactive C# REPL for Unity Editor and runtime Players.

Thank you for prompt_toolkit!