Support easy marking/unmarking of duplicate issues with new API's

[HowardWolosky]

Feature Idea Summary

GitHub allows you to mark an issue as a duplicate of another issue (as well as marking a pull request as a duplicate of another pull request) by adding the special phrase of Duplicate of #<num>. You can undo that marking of duplication by clicking "undo" on the timeline entry for that duplication notification.

We can ease this process for this module's users by exposing an API that automatically adds that Issue/PR comment directly to the item.

Feature Idea Additional Details

Initial thoughts on API structure:

Set-GitHubIssueAsDuplicate [[-OwnerName -RepositoryName] || [RepositoryUrl]] -Issue -DuplicateTo

In theory, Get-GitHubEvent should give us the ability to find the duplicate event for the "undo" scenario, but this still needs to be tested/verified. We also need to see what command that "Undo" button is actually sending, as I'm not seeing anything in the documentation about how to handle an undo.

Requested Assignment

I'm just suggesting this idea, but don't want to implement it.

Operating System

OsName               : Microsoft Windows 10 Pro
OsOperatingSystemSKU : 48
OsArchitecture       : 64-bit
WindowsVersion       : 2004
WindowsBuildLabEx    : 19041.1.amd64fre.vb_release.191206-1406
OsLanguage           : en-US
OsMuiLanguages       : {en-US}

PowerShell Version

Name                           Value
----                           -----
PSVersion                      5.1.19041.610
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.19041.610
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Module Version

Running: 0.16.0
Installed: