[New Feature]: Removal PRs should skip the Validation Pipeline

Description of the new feature/enhancement

A removal PR is where a manifest version is removed. To contrast with an addition PR, which adds a manifest version.

Example of a removal PR: https://github.com/microsoft/winget-pkgs/pull/138377/files

This would both conserve pipeline resources and also greatly reduce the time that removal PRs spend in pipeline. Removing a PR should skip most checks:

• No need to verify URL (static scan).
• No need to verify the manifest.
• No need to install and verify installer operation and application execution.
• No need to install and scan (dynamic scan).

Proposed technical implementation details (optional)

• Have some mechanism for detecting removal PRs.
  • One method is using the plus/minus ("+" or "-") which GitHub prepends to raw diffs:
    • If all of a file's lines start with minus "-", then the file is being removed.
    • If all files in the PR are being removed, then it's a removal PR.
  • Another method is to detect based on PR title. This is easier to implement but much more error-prone.
  • Other methods might exist. (Possibly in API calls)
• Have removal PRs be exempted from pipeline checks which are unnecessary for removal.

Start by tracing the Validation Pipeline and how it identifies pull-request changes; the issue names no files or tests. Determine how a manifest removal PR can be detected reliably, then verify that removal PRs skip URL, manifest, installer, application-execution, and dynamic-scan checks while other PRs retain their existing validation.