feat: add support for Dagger CUE LSP

[dolanor]

Hi,
we recently released a CUE LSP, and I followed vim-lsp documentation to configure it, and it works pretty well. But I wanted to make it even simpler to use it and started creating this PR.
But somehow, I can't make it work.
The configuration that works with bare vim-lsp is:

if executable('dagger')
    au User lsp_setup call lsp#register_server({
        \ 'name': 'dagger_cue_lsp',
        \ 'cmd': {server_info->['dagger', 'cuelsp']},
        \ 'allowlist': ['cue'],
        \ })
endif

function! s:on_lsp_buffer_enabled() abort
    setlocal omnifunc=lsp#complete
    setlocal signcolumn=yes
    if exists('+tagfunc') | setlocal tagfunc=lsp#tagfunc | endif
    nmap <buffer> gd <plug>(lsp-definition)
    nmap <buffer> gs <plug>(lsp-document-symbol-search)
    nmap <buffer> gS <plug>(lsp-workspace-symbol-search)
    nmap <buffer> gr <plug>(lsp-references)
    nmap <buffer> gi <plug>(lsp-implementation)
    "nmap <buffer> gt <plug>(lsp-type-definition)
    nmap <buffer> ,rn <plug>(lsp-rename)
    nmap <buffer> [g <plug>(lsp-previous-diagnostic)
    nmap <buffer> ]g <plug>(lsp-next-diagnostic)
    nmap <buffer> K <plug>(lsp-hover)

    let g:lsp_format_sync_timeout = 1000
    autocmd! BufWritePre *.cue call execute('LspDocumentFormatSync')

    " refer to doc to add more commands
endfunction

let g:lsp_log_file = expand('/tmp/vim-lsp.log')
let g:lsp_log_verbose = 1

let g:lsp_experimental_workspace_folders = get(g:, 'lsp_experimental_workspace_folders', 1)

augroup lsp_install
    au!
    " call s:on_lsp_buffer_enabled only for languages that has the server registered.
    autocmd User lsp_buffer_enabled call s:on_lsp_buffer_enabled()
augroup END

On a second improvement, I'd like to be able to use either the cuelsp server embedded in the dagger binary, but I want user who just want to use CUE not to have to download dagger. We have a standalone cuelsp binary as well. I think somehow haskell has 2 binaries as well, but I couldn't get the configuration right, so I reduced to just 1 case.

[mattn]

Thank you. LGTM. Could you please follow the indentation to other's code?