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

まず、language service が現在どのように dot-source 参照を解決しているかを追跡し、提案されている protocol request/response types とコアの PSES file-resolver interface を確認します。クライアントが自身の workspace 内の参照ファイルを解決し、サーバーがプロトコル経由でそれらを要求し、resolver の実装を実行時に選択できれば完了です。