Request referenced files from the client instead of attempting to load from the language service

Currently any files that are referenced from an edited script (i.e. dot-source references) are being resolved and loaded by the language service. Since we want to target remote editing scenarios (language service running in a web service), we need to shift the file resolution responsibility to the client so that it can resolve files with respect to its own workspace.

This will require the introduction of new protocol request/response types which allow the server to request files from the client. The core PSES API will handle this behavior through an interface so that file resolver implementations can be chosen at runtime.

This bug in vscode-powershell should benefit from the improvement: https://github.com/PowerShell/vscode-powershell/issues/31

首先跟踪 language service 当前如何解析 dot-source 引用，并查看提议的 protocol request/response types 以及核心 PSES file-resolver interface。完成的标志是 client 在自己的 workspace 中解析被引用的文件，server 通过 protocol 请求这些文件，并且可以在运行时选择 resolver 实现。