Fix CacheV2 task to ensure cache directory exists for yarn packages

[Copilot]

The Issue

The CacheV2 task was failing during the post-job execution phase when caching yarn packages. The error occurred because the SavePipelineCacheV0 agent plugin attempted to archive a directory that didn't exist:

Information, There is a cache miss.
tar: D\:\\a\\1\\.yarn: Cannot open: No such file or directory
tar: Error is not recoverable: exiting now
##[error]Process returned non-zero exit code: 2

This happens specifically on the first run when:

1. There is a cache miss (no previous cache to restore)
2. The directory specified in the path input doesn't exist
3. The post-job execution tries to save the cache but fails because the directory doesn't exist

The Solution

Added a JavaScript script that runs before the agent plugin to ensure the cache directory exists:

1. Created restoreCache.js that checks if the directory exists and creates it if needed
2. Modified task.json to include Node execution targets (10, 16, and 20) while preserving the agent plugin functionality
3. Added proper package.json for dependency management
4. Added README.md with documentation and usage examples

This ensures that when users set up caching for Yarn packages as described in the documentation:

variables:
  YARN_CACHE_FOLDER: $(Pipeline.Workspace)/.yarn

steps:
- task: Cache@2
  inputs:
    key: 'yarn | "$(Agent.OS)" | yarn.lock'
    restoreKeys: |
       yarn | "$(Agent.OS)"
    path: $(YARN_CACHE_FOLDER)
  displayName: Cache Yarn packages

The task will now work properly even on the first run when the directory doesn't exist yet.

Fixes #12892.

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