fix: build a single engine for multiple projects using the same toolchain versions

[doorgan]

Fix #466

When resuming a session on Expert itself, I would regularly see issues due to multiple engine builders trying to modify the same files. This PR introduces a coordinator that keeps track of finished and in-flight builds, and ensures that:

• An engine for an elixir+otp pair is built once during the lifetime of the Expert instance, and reused upon project restarts
• If multiple projects using the same versions request an engine at the same time, or while that version is being built, they all wait for the same, single builder to finish instead of starting new ones
• Additionally, ensures engines for different versions can be built concurrently

Besides fixing that issue, it also cuts down startup time for Expert by a lot, since we builds the engine once instead of thrice(forge, engine, expert), once one engine finishes building the rest are instant.