Throw a warning when I redefine a global variable in a local scope

Like the title says, it would be useful if I was warned whenever I try to reuse a global variable's name in a more local scope.

$a = 'Unchanged'
function foo {
    Write-Host "foo can read `$a : `$a = $a"
    $a = 'Changed from foo' # this should be warning!!
    Write-Host "foo can change it's own copy of `$a: `$a = $a"
}
foo
Write-Host "After foo call: `$a = $a"

Thanks to @vors for the example.

以 issue 中的 PowerShell 範例作為行為起點。檢視 PSScriptAnalyzer 如何回報與範圍相關的診斷，然後定義警告觸發條件，並使用所示範的全域變數和區域變數案例進行驗證。