feat: More responsive diagnostics

[WillLillis]

Original discussion in #184 (thanks, @ldelossa!)

Currently asm-lsp offers diagnostics on save. Many other LSPs offer diagnostics as users edit the file, regardless of writes to disk. This should be doable by syncing a user's edits to a "shadow" copy of the buffer in question. The appropriate compile command can be run on this shadow copy. I'm planning on adding this as an opt-in, experimental feature initially because there's so many unknowns. Some initial thoughts on what this will require:

• Finding or implementing a basic request queue to service compile commands in a separate thread.
  • If a request is sitting in the queue, we don't need to push a duplicate request.
• Integration tests covering diagnostics support before development of this feature starts
  • Separate testing is needed for Windows because of how weird paths are treated in that OS
• The current diagnostics implementation is rather buggy. (Diagnostic not displaying on Windows #173, bug: unable to enable diagnostics with 'compile_flags.txt' #138, asm-lsp not displaying error diagnostics in Neovim for .s files #115, and likely other issues). Adding this functionality would be a good time throw away this initial implementation and simplify things as much as possible.
• Live reloading of .asm-lsp.toml and build files (compile_commands.json and compile_flags.txt)

I'm planning on implementing this after adding all of the currently outstanding language additions. The language additions will come either in 0.10.0 or 0.10.1.