Preserve original `list-position`, when local/unavailable songs are skipped

[dxrcy]

Title

Preserve original list-position, when local/unavailable songs are skipped

Description

When downloading with {list-position} included in the --output option. When a local song is ignored, or a song is unavailable, previously the track numbers would shift down to fill the gap; now the track numbers correspond to the list positions in the original playlist.

This pull request is unfinished, as I need feedback on whether this is even a good addition to the project, and whether a new CLI option should be added :)

Related Issue

• Resolves #2104

Motivation and Context

I think that an option to preserve the original number would be beneficial, especially if a user wants to manually include the local tracks, so they won't have to rename the files to fix the numbers.

How Has This Been Tested?

Since this is such a small change, I just ran some manual tests and checked compilation still worked.

Types of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] My code follows the code style of this project
• [x] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly
• [x] I have read the CONTRIBUTING document
• [x] I have read the CORE VALUES document
• [ ] I have added tests to cover my changes
• [ ] All new and existing tests passed