Improve performance for large workspaces

[rvanasa]

Implements the following optimizations:

• Limits the type checker to the open tab and canister entry points (rather than the entire workspace).
• Increases delay before checking workspace on file save
• Caches diagnostics to reduce number of LSP messages
• Refactors for non-blocking file I/O

[rvanasa]

Closing in favor of a minimal subset of these optimizations which offer the biggest performance improvements.