Add "Merge" option for long press on created playlist

[SameenAhnaf]

Checklist

• I checked, but didn't find any duplicates (open OR closed) of this issue in the repo.
• I have read the contribution guidelines given at https://github.com/TeamNewPipe/NewPipe/blob/HEAD/.github/CONTRIBUTING.md.
• This issue contains only one feature request. I will open one issue for every feature I want to request.

Describe the feature you want

Sometimes, others' playlists stay splitted and we want to merge them into one single playlist. Remember that newly merged playlist should be regarded as a "Created playlist".

Merge: A new popup should appear which will have upper and lower menu. The lower menu should show bookmarked and created playlists. The upper menu should show the playlists to be merged. Playlists should appear from lower menu to upper menu by clicking on them once. The playlists in upper menu should be rearrangable and removable (just like videos on enqeued list). Clicking on "OK" will ask for new playlists' name. First playlists' name could be kept already mentioned for user convenience.

A checkbox for "Save previous playlists" can be added below the rename title option and above OK & Cancel button to save or delete the playlists which were supposed to be merged. Checking the checkbox should save playlists and unchecking should delete previous playlists. An extra confirmation popup is not required for this action.

Split option is not necessary as the playlist has to be open anyway. Bulk selection gestures (mentioned in #5657) should be added for easy splitting.

Is your feature request related to a problem? Please describe it

Additional context

How will you/everyone benefit from this feature?