Add API proposal version check

[alexr00]

No description provided.

[Copilot]

Pull request overview

Adds a new GitHub Actions workflow to detect and gate API proposal // version: changes in src/vscode-dts/vscode.proposed.*.d.ts files, aiming to discourage unnecessary proposal version bumps and require explicit acknowledgement when they occur.

Changes:

• Introduces API Proposal Version Check workflow triggered on PRs that touch proposed API d.ts files.
• Detects proposal // version: changes, posts/updates a PR warning comment, and fails the check unless an override comment is present.
• Adds an issue_comment trigger intended to allow unblocking via a slash command.

[github-actions]

⚠️ API Proposal Version Change Detected

The following proposed API files have version changes: vscode.proposed.chatDebug.d.ts

API proposal version changes should only be used when maintaining compatibility is not possible. Consider keeping the version as is and maintaining backward compatibility.

Any version changes must be updated by the consuming extensions before the next insiders for the extension to work.

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If the version change is required, comment /api-proposal-change-required to unblock this check and acknowledge that you will update any critical consuming extensions (Copilot Chat).