Call hierarchy for targets

Implement `textDocument/prepareCallHierarchy`, `callHierarchy/incomingCalls`, and `callHierarchy/outgoingCalls` for make targets. Incoming calls would show which targets list the current target as a prerequisite, and outgoing calls would show the current target's prerequisites. This helps users understand complex dependency chains in large Makefiles.

Start by locating the LSP handlers for textDocument/prepareCallHierarchy, callHierarchy/incomingCalls, and callHierarchy/outgoingCalls, then inspect how Make targets and prerequisites are represented. Done means all three methods return call hierarchy data showing prerequisite-to-target relationships in both directions for Make targets.

Written by the indexing model from the issue text.