`PSUseDeclaredVarsMoreThanAssignments` and assigning multiple variables

Aside from the false positive issues with the PSUseDeclaredVarsMoreThanAssignments rule, it has also a false negative bug:

This concerns assigning multiple variables:

This (as the only script line), correctly reports a PSUseDeclaredVarsMoreThanAssignments volitation on $a:

$a = $null

But this doesn't:

$a, $b = $null

PSScriptAnalyzer version: 1.24.0

Name                           Value
----                           -----
PSVersion                      7.5.4
PSEdition                      Core
GitCommitId                    7.5.4
OS                             Microsoft Windows 10.0.26100
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

首先使用兩個 PowerShell 程式碼片段重現問題，然後追蹤 PSUseDeclaredVarsMoreThanAssignments 規則對多變數指派的處理。為回報的漏報新增回歸涵蓋，並驗證未使用的變數是否如預期被回報。