Allow customization of code formatting rules

In this block of code: https://github.com/PowerShell/PowerShellEditorServices/blob/master/src/PowerShellEditorServices.Protocol/Server/LanguageServerSettings.cs#L209

The list of PSScriptAnalyzer rules used to perform the Formatting function is hard-coded. It would be nice to have a way of extending this list to include custom rules. For example, I have one that removes any trailing whitespace from lines, but it doesn't run if I use VSCode's "Format Document" or "Format Selection" actions.

從 src/PowerShellEditorServices.Protocol/Server/LanguageServerSettings.cs 中第 209 行附近的格式化規則清單開始。追蹤 Format Document 和 Format Selection 動作如何使用該清單，接著判斷應如何提供自訂 PSScriptAnalyzer 規則，並將其納入兩條路徑。完成的標準是，使用者可以擴充格式化規則，且其自訂規則會透過這些編輯器動作執行。