fix(tasks): update tasks appendix schema

[lizuju]

Summary

• mark task type and command as optional in the tasks appendix schema snippet
• document object-form background beginsPattern/endsPattern via a WatchingPattern interface

Fixes microsoft/vscode#243556

Verification

• Checked the updated appendix for optional task fields and object-form background patterns with a local Node assertion.
• Compared against microsoft/vscode source: problemMatcher.ts defines beginsPattern?: string | IWatchingPattern and endsPattern?: string | IWatchingPattern; jsonSchema_v2.ts requires only label for the generic task description and adds command/type only for process tasks.
• Ran git diff --check.

[lizuju]

@microsoft-github-policy-service agree

[ntrogh]

Thanks