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.

从 src/PowerShellEditorServices.Protocol/Server/LanguageServerSettings.cs 中第 209 行附近的格式化规则列表开始。跟踪 Format Document 和 Format Selection 操作如何使用该列表，然后确定应如何提供自定义 PSScriptAnalyzer 规则并将其纳入两条路径。完成的标准是，用户可以扩展格式化规则，并且其自定义规则会通过这些编辑器操作运行。