Background compilation

LazyVerilog reports diagnostics in two ways:

1. Fast diagnostics on every edit: syntax, preprocessor, and lint.
2. Background compilation, off by default: a full slang elaboration that adds semantic diagnostics such as unknown modules, width mismatches, and type errors.

[compilation]
background_compilation = true

It can be slow on a weak machine, so enable it where you have the CPU and memory to spare.

How it behaves

• It starts 1.5 seconds after you stop typing, and only the newest edit is compiled.
• It compiles the files in [design].vcode plus your open buffers, with your unsaved text, using the project's own define and +incdir+ entries.
• Each open project is compiled on its own, one after another, so memory stays at one project's worth. The setting is read per project.
• Results appear for open files only. Requests such as hover never wait for it.
• The worker runs at low priority on Linux. Worker count and the 1.5 second delay are fixed.