🩹 [Patch]: Add Base64Url string conversion functions

[Copilot]

This PR adds comprehensive Base64Url string conversion functionality to the module, providing URL-safe variants of the existing Base64 functions.

What's Added

New Functions

ConvertTo-Base64Url - Converts strings to base64url encoding

• Replaces + with - and / with _ for URL safety
• Removes padding = characters
• Supports pipeline input and multiple encoding options
• Includes alias ConvertTo-Base64UrlString

ConvertFrom-Base64Url - Decodes base64url strings

• Handles URL-safe characters (- and _)
• Automatically adds padding when needed for proper decoding
• Validates input using Test-Base64Url function
• Includes alias ConvertFrom-Base64UrlString

Test-Base64Url - Validates base64url strings

• Checks for valid character set (A-Z, a-z, 0-9, -, _)
• Rejects standard base64 characters (+, /, =)
• Returns boolean validation result

Usage Examples

# Convert to Base64Url (URL-safe, no padding)
"Hello World" | ConvertTo-Base64Url
# Output: SGVsbG8gV29ybGQ

# Convert from Base64Url
"SGVsbG8gV29ybGQ" | ConvertFrom-Base64Url  
# Output: Hello World

# Validate Base64Url strings
"SGVsbG8gV29ybGQ" | Test-Base64Url        # True (valid)
"Invalid+String/=" | Test-Base64Url       # False (contains +, /, =)

# Round-trip conversion maintains data integrity
$original = "Test string with special chars: +/?="
$encoded = $original | ConvertTo-Base64Url
$decoded = $encoded | ConvertFrom-Base64Url
$original -eq $decoded  # True

Key Features

• URL-Safe: Uses - and _ instead of + and /
• No Padding: Removes = padding characters for cleaner URLs
• Pipeline Support: Full ValueFromPipeline compatibility
• Multiple Encodings: Supports UTF8, ASCII, Unicode, etc.
• Input Validation: Robust validation with helpful error messages
• Consistent API: Follows the same patterns as existing Base64 functions

Testing

Added 21 comprehensive test cases covering:

• Basic conversion functionality
• Pipeline input handling
• Array processing
• File content processing
• URL-safe character validation
• Round-trip conversion integrity
• Error handling for invalid inputs

All 35 tests pass (14 existing + 21 new).

Fixes #10.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Marius Storhaug (@MariusStorhaug) 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.