IDE: Faster workspace/symbol

[jvilk-stripe]

Problem

workspace/symbol is slow on large projects. It attempts to support IntelliJ-style prefix searches and orders results as such, but we do not control the order of the results -- VS Code reorders results itself.

Proposed solution

So long as it is cheaper to compute, support a simpler substring search instead. For Stripes tackling this project: It may be worth mining Stripe engineer logs to verify that users are not expecting the IntelliJ-like behavior.