Investigate improving configuration flow by sending workspace/configuration at startup

[fwcd]

Extracted from #367 (comment)

There was some discussion upstream (in the LSP repo) on encouraging servers to move to a pull-model where the server fetches configurations through workspace/configuration at startup instead of relying on the configuration to always be passed at launch. Ideally we should probably do something similar in the medium term.

Some interesting threads:

• Can I rely on didChangeConfiguration to be always called after server initialization? microsoft/vscode-languageserver-node#524
• Spec should discourage abuse of initializationOptions and didChangeConfiguration microsoft/language-server-protocol#567
• HLS does not send workspace/configuration on startup haskell/haskell-language-server#2762