Add workflow to automatically upgrade extension npm dependencies

[Copilot]

This PR adds a new GitHub Actions workflow that automatically upgrades npm dependencies for the VS Code extension located in the extension/ folder.

Workflow Features

The workflow runs weekly on Mondays at 8am PST (same schedule as other dependency workflows) and:

• Installs current dependencies using yarn install --frozen-lockfile to ensure a clean starting point
• Runs yarn upgrade to update all packages to their latest compatible versions
• Handles failures gracefully using continue-on-error: true to continue when some packages are unavailable in the npm registry
• Detects changes by comparing package.json and yarn.lock files before and after the upgrade
• Creates a pull request only when actual changes are detected, using the same PR creation action as other automation workflows
• Includes comprehensive logging to help troubleshoot any packages that fail to upgrade

Error Handling for NPM Feed Issues

The workflow addresses the requirement to handle cases where some packages may not be available in the npm feed by:

1. Using continue-on-error: true on the yarn upgrade step so the workflow continues even if some packages fail
2. Capturing upgrade output to a log file for analysis
3. Checking for and reporting any errors encountered during the upgrade process
4. Only creating PRs when successful upgrades actually change the dependency files

Implementation Details

The workflow follows existing repository patterns:

• Uses the same scheduling as update-dependencies.yml (weekly on Mondays)
• Includes proper repository owner checks (github.repository_owner == 'dotnet')
• Uses appropriate permissions (contents: write, pull-requests: write)
• Uses the same PR creation action (dotnet/actions-create-pull-request)
• Follows consistent commit message and PR title formatting with [Automated] prefix

The generated PRs will include clear documentation explaining the automation and noting that some packages may have failed to upgrade due to npm registry availability.

Fixes #10855.

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[davidfowl]

@joperezr isn't this what dependabot is for 😄