feat(zls): add vim.lsp.config support

[nkhlmn]

Disclaimer: i'm a zig newb and i'm not up to speed on the planned direction for nvim and vim.lsp.config. Would appreciate some review to make sure i'm not overlooking anything. This does seem to work fine in my limited testing though i.e. making sure the lsp starts up in some random projects and the functionality works, and that checkhealth vim.lsp looks fine.

Includes the following changes from the legacy config:

• Use root_markers instead of root_dir
  • This removes the dependency on util
• Remove on_new_config
  • I wasn't sure what/if to replace this. I took a look at what was done for omnisharp, which was listed in Missing legacy configs #3705 and has been ported/merged and it doesn't appear to have an equivalent there so I am assuming it's fine to leave this out
• Replace single_file_support = true with workspace_required = false