Add datetime formatting capability to variable set task

[Copilot]

This PR adds the ability to format the current date and time according to a user-provided format string in the vsts-variable-set task.

Changes Made

New Task Input Options

• Added "current date/time" option to the "From" dropdown in the vsts-variable-set task (v3)
• Added "Date/Time Format" input field with intelligent default: yyyy-MM-dd HH:mm:ss
• The format field appears only when "From" = "datetime" for a clean UI experience

Format Pattern Support

The implementation supports common datetime format patterns:

• yyyy - 4-digit year (2025)
• yy - 2-digit year (25)
• MM - 2-digit month (07)
• dd - 2-digit day (15)
• HH - 24-hour format (14)
• hh - 12-hour format (02)
• mm - 2-digit minute (30)
• ss - 2-digit second (45)
• tt - AM/PM indicator

Usage Examples

# Basic timestamp variable
- task: VariableSetTask@3
  inputs:
    variableName: 'BuildTimestamp'
    from: 'datetime'
    dateTimeFormat: 'yyyy-MM-dd HH:mm:ss'
  # Result: 2025-07-15 14:30:00

# Build number with datetime
- task: VariableSetTask@3
  inputs:
    variableName: 'Build.BuildNumber'
    from: 'datetime'
    dateTimeFormat: 'Build-yyyy.MM.dd.HHmm'
  # Result: Build-2025.07.15.1430

# Short date for versioning
- task: VariableSetTask@3
  inputs:
    variableName: 'VersionDate'
    from: 'datetime'
    dateTimeFormat: 'yyyyMMdd'
  # Result: 20250715

Azure Pipelines Integration

• ✅ Works with Build.BuildNumber (updates build number)
• ✅ Works with Release.ReleaseName (updates release name)
• ✅ Works with custom variables (creates new pipeline variables)
• ✅ Supports secret variables and output variables
• ✅ Compatible with both tasklib and direct VSO commands
• ✅ Maintains full backward compatibility

Technical Implementation

• Minimal changes: Only modified vsts-variable-set/v3/task.json and vsts-variable-set/v3/vsts-variable-set.ts
• Robust pattern matching: Uses ordered regex replacements to avoid conflicts with regular text
• Clean error handling: Graceful fallbacks and sensible defaults
• Documentation: Clear help text with examples in the task UI

This enhancement enables Azure Pipelines users to generate datetime-formatted variables for build numbers, timestamps, file naming, and custom workflow needs without requiring external tools or complex scripts.

Fixes #776.

---

💡 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.