fix(zls): use workspace zls.json if available

[lawrence-laz]

Language server config is read from zls.json. By default common global and user config directories are searched. However, this omits per-project configuration, which can be available in the workspace dir.

We already look for zls.json to determine root dir, but we don't use it when starting the language server.

This change appends cmd with --config-path "zls.json", which uses the file when it's available and fall-backs to default logic when file is not available.

[glepnir]

why we don't check zls.json first build a cmd

[lawrence-laz]

We don't really have to, since zls goes back to using global config if a specified config file does not exists, but we can.

Would you rather have cmd as a function that checks if zls.json exists in workspace and and then generates command based on that?

[glepnir]

This is fine if the server can fallback. Just wondering if it might bother some newbies.

[lawrence-laz]

I tried looking into how to provide a function for default_config.cmd, but couldn't find it, does it always have to be a table?

[glepnir]

nope it can be a function.

[lawrence-laz]

I'm struggling a bit here.
Tried providing a function instead of a table and printed the arguments, which got me:

{
  notification = <function 1>,
  on_error = <function 2>,
  on_exit = <function 3>,
  server_request = <function 4>
}

I then tried looking for these function names in source, but got unrelated stuff or in case of server_request even no results at all.
Tried looking through docs again, but they only seem to contain an example for table as well:

- {cmd} `list[string]`

  a list where each entry corresponds to the blankspace delimited part of
  the command that launches the server. The first entry is the binary used
  to run the language server. Additional entries are passed as arguments.

  The equivalent `cmd` for: 
 
  foo --bar baz
 
  is:
 
  {'foo', '--bar', 'baz'}

So not sure how to do this.
Or should I use on_setup hook instead?
Could you give some pointers on how this could be implemented?

[glepnir]

 cmd = function() xxxxxx end