Support multi-root workspaces

[valentjn]

Window-wide settings are not supported for the new multi-root workspaces. Some work has to be done to support these. They can also be nested.

Resources:

• https://github.com/microsoft/vscode/wiki/Adopting-Multi-Root-Workspace-APIs: Describes the changes in the API.
• https://github.com/microsoft/vscode-extension-samples/tree/master/lsp-sample: Gives an example of configuration settings in an LS.
• https://github.com/Microsoft/vscode-extension-samples/tree/master/lsp-multi-server-sample: Shows how to have one LS per workspace.

[valentjn]

LT_EX now supports this in version 4.7.0. But the implementation was more complicated than expected...

For every LT check, we need to have the current configuration (language, dictionary, ...) for the file to be checked. This configuration can differ from workspace folder to workspace folder. However, I'm not sure when LSP clients notify the language server that the configuration has been changed (global configuration, workspace configuration, or workspace folder configuration?). The LSP spec doesn't say anything about that. So now we actively request the configuration from the client at the beginning of every LT check.

In addition, a single language server now has to check documents of different languages, for instance, we could have an English and a German workspace folder. So now we cache LanguageTool instances for the different languages, which (further) increases memory usage, but keeps checking time short. Also, I don't know when exactly a new instance of the server gets spawned. It doesn't seem to be once per VS Code window...

Please open new issues if there are any problems with the implementation (which is likely, given the amount of uncertainty I have).