LSP: Implement type hierarchy protocol extension

[jvilk-stripe]

Problem

There's no way to query LSP for the type hierarchy (e.g., subtypes and supertypes of a given type).

Proposed solution

Implement the proposed type hierarchy protocol extension:
microsoft/vscode-languageserver-node#426

clangd and a Java LSP server both implement it, so it has traction.

Note that this is a low-priority feature request. VS Code does not implement this part of the protocol so we would have to implement a UI for it in the extension.