Remove the need for nvim/nvim-lspconfig

[ckipp01]

I still need to clean up a few things and pretty much replace all of the documentation/readme, but this is the approach that I'd like to take. To summarize, this pr will get rid of the need for a user to have nvim/lsp-config installed. They still can if they want to, but this should become the preferred way to use Metals with nvim lsp mainly because of all the extra functionality and Metals-specific lsp extensions it provides. The main idea is that instead of having a user add in a bunch of handlers on their own and also have to keep adding them in as we add them, is for them to simply have the following:

if has('nvim-0.5')
  augroup lsp
    au!
    au FileType scala lua require('metals').initialize_or_attach({})
  augroup end
endif

This will basically provide all of the functionality that nvim/nvim-lspconfig and this extension provided before, but with way less work to setup. However, this still remains very customizable. For example, my personal setup locally looks like this:

:lua << EOF
  metals_config = {}
  metals_config.handlers = {}
  metals_config.init_options = {}

  metals_config.on_attach = function()
    require'completion'.on_attach();
  end

  metals_config.init_options.statusBarProvider = 'on'

  metals_config.handlers["textDocument/publishDiagnostics"] = vim.lsp.with(
    vim.lsp.diagnostic.on_publish_diagnostics, {
      virtual_text = {
        prefix = '',
      }
    }
  )
EOF

if has('nvim-0.5')
  augroup lsp
    au!
    au FileType scala lua require('metals').initialize_or_attach(metals_config)
  augroup end
endif

This allows me to use the on_attatch() from nvim-completions, change the default show-message to on to be able to use metals/status, and also customize how my diagnostics look.

This plugin also will fully handle the installation of Metals for the user by providing a :MetalsInstall command to put Metals in a specific location and use it. I've tried to be quite descriptive with messages to help the user know what's going on.

I still have some further things to do, but this will at least give an idea of the direction I'm headed.

@landerlo feel free to take a peak at this and give me any feedback.

TODO

• Update readme with new instructions (keep in short, and focus on the help docs instead)
• Update the metals.txt with basically all the information a user would need
• Add in a :MetalsInfo command to give some practical info, basically the output of metals --version which shows the version of metals and also the supported Scala Versions. This command can also be used to show where Metals is installed
• Fix CI -- figure out if I just want to use lua-format instead of luacheck
• Figure out what I should do with the set shortmess-=F. This needs to be set in order for the FileType autocmd to display print, which is needed for the helpful messages.

[ckipp01]

For anyone that stumbles onto this, you can find a short migration guide for these changes here: #17 (comment)