Add a sample for Encode / Decode Base64

[veler]

This PR adds a new Encode / Decode Base64 sample.

I'm considering adding selection restoration. Looking with @olegtk on how it works.

[Copilot]

PR Overview

This PR adds a new sample extension that demonstrates how to encode and decode Base64 text within Visual Studio. The key changes include:

• Adding an extension entry point (EncodeDecodeBase64Extension.cs) with metadata registration.
• Introducing a command (EncodeDecodeBase64Command.cs) that processes selected text by encoding or decoding it between plain text and Base64.
• Providing a README.md that explains the sample’s functionality, command definition, usage, and relevant guidelines.

Reviewed Changes

File	Description
New_Extensibility_Model/Samples/EncodeDecodeBase64/EncodeDecodeBase64Extension.cs	Registers the extension with proper metadata for Visual Studio.
New_Extensibility_Model/Samples/EncodeDecodeBase64/README.md	Documents the sample usage, command definition, and walkthrough.
New_Extensibility_Model/Samples/EncodeDecodeBase64/EncodeDecodeBase64Command.cs	Implements the command logic that encodes or decodes Base64 text from the active editor.

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

New_Extensibility_Model/Samples/EncodeDecodeBase64/EncodeDecodeBase64Command.cs:77

• Allocating a buffer using stackalloc based on the input text length may cause a stack overflow for very large inputs. Consider validating the input size or switching to heap allocation for longer texts.

Span<byte> buffer = stackalloc byte[text.Length];