Script to mark API as shipped (dotnet#4837)

eng/ApiCompatibility/README.md

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+### Mark Shipped Tool
+========
+
+This tool should be run after every supported release that has API changes. It will 
+merge the collection of PublicApi.Shipped.txt files with the PublicApi.Unshipped.txt 
+versions.  This will take into account `*REMOVED*` elements when updating the files.
+
+Usage:
+
+``` cmd
+mark-shipped.cmd
+```

eng/ApiCompatibility/mark-shipped.cmd

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+@echo off
+powershell -noprofile -executionPolicy RemoteSigned -file "%~dp0\mark-shipped.ps1"

eng/ApiCompatibility/mark-shipped.ps1

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+[CmdletBinding(PositionalBinding=$false)]
+param ()
+
+Set-StrictMode -version 2.0
+$ErrorActionPreference = "Stop"
+
+function MarkShipped([string]$dir) {
+    $shippedFilePath = Join-Path $dir "PublicAPI.Shipped.txt"
+    $shipped = Get-Content $shippedFilePath
+    if ($null -eq $shipped) {
+        $shipped = @()
+    }
+
+    $unshippedFilePath = Join-Path $dir "PublicAPI.Unshipped.txt"
+    $unshipped = Get-Content $unshippedFilePath
+    $removed = @()
+    $removedPrefix = "*REMOVED*";
+    Write-Host "Processing $dir"
+
+    foreach ($item in $unshipped) {
+        if ($item.Length -gt 0) {
+            if ($item.StartsWith($removedPrefix)) {
+                $item = $item.Substring($removedPrefix.Length)
+                $removed += $item
+            }
+            else {
+                $shipped += $item
+            }
+        }
+    }
+
+    $shipped | Sort-Object | ?{ -not $removed.Contains($_) } | Out-File $shippedFilePath -Encoding Ascii
+    $null | Out-File $unshippedFilePath -Encoding Ascii
+}
+
+try {
+    Push-Location $PSScriptRoot\..\..
+
+    foreach ($file in Get-ChildItem -re -in "PublicApi.Shipped.txt") {
+        $dir = Split-Path -parent $file
+        MarkShipped $dir
+    }
+}
+catch {
+    Write-Host $_
+    Write-Host $_.Exception
+    exit 1
+}
+finally {
+    Pop-Location
+}