Language server async overhaul

[micahscopes]

What was wrong?

The tower-lsp based prototype was full of issues. See #979 for details.

This version is way better. In addition to the architecture overhaul, this PR includes a bunch of features and stability fixes.

Changes

• Big architecture overhaul, switched from tower-lsp to async-lsp
  • much better control over concurrency and event processing order, working around issues described here
  • custom actor implementation supporting lock-free language server state and handlers with thread-local futures
• Optional stream-based event handling support
• Multithreaded tracing and logging improvements
• Debounced, multithreaded diagnostics
• TCP support; support for multiple clients
• Organize and prepare server architecture for a variety of LSP features
• Implement a task queue system for long-running tasks
• Enhance hover feature to return useful information and documentation
• Initial work on VSCode extension release pipeline

[sbillig]

The new code looks nice! Looking forward to using this.