Add to nvim-lspconfig

[checkraisefold]

https://github.com/neovim/nvim-lspconfig

It's pretty simplistic to make a configuration, but I'm not familiar with dm-langserver at all and don't know Rust so I can't properly make it. I don't know the proper capabilities. If a maintainer wants to pick this up, here's the barebones functionality server config I made:

local util = require 'lspconfig.util'

local bin_name = 'dm-langserver'
local cmd = { bin_name }

if vim.fn.has 'win32' == 1 then
  cmd = { 'cmd.exe', '/C', bin_name }
end

return {
  default_config = {
    cmd = cmd,
    filetypes = { 'dm', 'dme' },
    root_dir = util.root_pattern('SpacemanDMM.toml', '.git'),
  },
  docs = {
    description = [[
https://github.com/SpaceManiac/SpacemanDMM/tree/master/crates/dm-langserver

Language Server for BYOND DreamMaker code

`dm-langserver` can be installed via cargo:
cargo install --git https://github.com/SpaceManiac/SpacemanDMM.git dm-langserver
]],
  },
}

In addition, it looks like DM files require a filetype autocmd for the LSP to work properly.

vim.cmd [[ autocmd BufNewFile,BufRead *.dm,*.dme set filetype=dm ]]

[checkraisefold]

It might also be worth making syntax highlighting for neovim if anyone is interested, since the only syntax highlighting right now is from 2016 and kind of sucks in comparison to vscode langclient syntax highlighting
https://github.com/ccraciun/vim-dreammaker