3131: vscode: simplified config and to removed one source of truth of default values r=matklad a=Veetaha
Though not intended initially, the implementation of config design is alike [dart's one](https://github.com/Dart-Code/Dart-Code/blob/master/src/extension/config.ts) as pointed by @matklad in PM.
Co-authored-by: Veetaha <gerzoh1@gmail.com>
Instead only opt-in to CallHierarchy since it has a vscode API but LSP support
is still proposed.
Discovered while working on SemanticTokens which does not have a vscode API
and is still in the proposed state. Somehow enabling it would crash the
language server.
See https://github.com/microsoft/vscode-languageserver-node/issues/572
