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

Beginne damit nachzuverfolgen, wie der Sprachdienst derzeit dot-source-Referenzen auflöst, und prüfe die vorgeschlagenen protocol request/response types sowie das zentrale PSES file-resolver interface. Als erledigt gilt die Aufgabe, wenn der Client referenzierte Dateien in seinem eigenen Workspace auflöst, der Server sie über das Protokoll anfordert und Resolver-Implementierungen zur Laufzeit ausgewählt werden können.