Project management

[nickspoons]

This PR is where I'm tracking modifications to project management. Done so far:

• Refactor project listening into new autoload script
• Maintain a list of projects which have been loaded for the solution (when a solution exists)

The list of projects can be used to display "server loading progress" in the statusline - here's an example using vim-sharpenup, PR#4:

Things I hope to achieve here:

• Detect whether the server is the OmniSharp-rolsyn HTTP server when stdio was expected, so we can warn the user, then change mode. This will allow us to make stdio the default in a backwards-compatible way.
  Update: I looked into restarting the server in HTTP mode, but it requires too many changes for little gain. The entire session is set up either for HTTP or stdio including autocmds for linting engines and compatibility checks. Detecting the wrong server and stopping it cleanly with a warning message is fine.
• Improve the load system so that highlighting/completion etc. become available as soon as the current project is loaded. Diagnostics will still warn of unloaded references, and definitions in unloaded projects will not be available until those projects are loaded etc.

Note: the final point hopefully encompasses a fix for #599 Update: it does not, #599 is a WSL bug and will be fixed on the OmniSharp-roslyn side.

Closes #468

Update: Since looking into the project loading mechanisms involves heavy use of the logs, I have included some other updates here that make this a bit more manageable:

• Give each OmniSharp-roslyn session its own log file. This means that a single vim session may be writing to multiple logs, if it is running multiple sessions. The old "stdio.log" file is still used as a global catchall - the filename of each session log is added to it, so it is possible to gf into actual log files from there.
• Format the log in a similar way to the VSCode formatting, making it actually readable - the raw JSON is extremely difficult to find relevant information in.
• Add new filetype and syntax for the logs, including folding raw requests/responses (when logging at debug level).

[nickspoons]

Status update: 4f2e069 is a very different approach to project loading, and works really well - highlighting, for example, is available very quickly, and no errors are received from the server. Request-replaying also works much better, as pending requests are saved per-buffer, instead of per-command, meaning that 2 buffers can have pending highlighting requests replayed as soon as their buffers are initialised.

However there are some diagnostic timeouts happening on the server which I believe are new, and diagnostics appear to take longer now than before. So I'll look into this before merging.