provide a way to override the server's working directory

[doudou]

The extension starts the server in the first folder in the workspace. I'm in a situation where I develop many interlinked ruby packages, and don't want to add the root directory where all these packages are located (it really does not work well in vscode, it's better to have each package added separately).

So, I'd want to have a configuration parameter to tell where the server should be started. I tried to do it myself, but then didn't manage to run the extension under vscode (see #178)

[castwide]

Could this be solved by opening a multi-root workspace? Solargraph will treat each workspace as a separate project with its own maps.