Support multi-root workspaces in vscode with different definitions files in each root

[chrissimon-au]

Related to #38, but on that one it was decided to just support a single shared definitions file.

To support separate definitions files, a few changes are needed:

1. LanguageServer needs to request configuration with a resource scope for the file currently being worked on (auto-complete or hover)
2. LanguageClient middleware added for #38 needs to detect the resource scope and skip the path rewriting. VsCode default configuration should supply the config value for the appropriate workspace based on the resource scope
3. LanguageServer needs to maintain a definitions file state for all possible definitions files and to use the correct one depending on the file being worked on