Use System.CommandLine for CLI parsing

[baronfel]

I've been using S.CL a lot on the .NET SDK, and while it's not quite as feature-ful as Argu it is way faster, and our use of Argu isn't super complex anyway.

S.CL also encourages a layering of options such that I can put together some middleware to take care of different slices of the startup code (like logging configuration) and get it out of the way of the main app startup code.

Anyway, seems to be working locally. Help output for the CLI looks good:

Description:
  An F# LSP server implementation

Usage:
  fsautocomplete [options]

Options:
  -v, --debug, --verbose                                 Enable verbose logging. This is equivalent to --log-level debug.
  --attach-debugger                                      Launch the system debugger and break immediately
  -l, --log-file, --logfile <log-file>                   Send log output to specified file.
  --filter <filter>                                      Filter logs by category. The category can be seen in the logs inside []. For example: [Compiler].
  --wait-for-debugger                                    Stop execution on startup until an external debugger to attach to this process
  --host-pid, --hostPID <host-pid>                       Process ID of the host process.
  --background-service-enabled                           Enable running typechecking services in a background process. Enables various performance optimizations.
  --project-graph-enabled                                Enable MSBuild Graph workspace loading. Should be faster than the default, but is experimental.
  --log-level <Debug|Error|Fatal|Information|Verbose|Warning>  Set the log verbosity to a specific level.
  --version                                              Show version information
  -?, -h, --help                                         Show help and usage information

One thing that's useful is testing the parsing of your CLI app, which you can do with the [parse] directive. This lets you see how the command would be parsed so you can make sure that nothing erroneous happens

17:02:47 ❯ dotnet run -- [parse] --filter Compiler LSP FileSystem "a log source with spaces"
[ fsautocomplete [ --filter <Compiler> <LSP> <FileSystem> <a log source with spaces> ] ]

Here, the command fsautocomplete is given the option --filter, and that option has 4 arguments. This is nicer (and more capable!) parsing code than we had for filters before IMO.

[baronfel]

As part of this I dropped the --host-pid parameter, since it was not used in the codebase anymore