refactor(diagnostic): clean up DiagnosticCollection.set() (#1688)

Author: Ferdi265

autoload/coc/util.vim

@@ -325,10 +325,14 @@ function! s:variables(bufnr) abort
   return variables
 endfunction
 
-function! coc#util#root_patterns()
+function! coc#util#root_patterns() abort
   return coc#rpc#request('rootPatterns', [bufnr('%')])
 endfunction
 
+function! coc#util#get_config(key) abort
+  return coc#rpc#request('getConfig', [a:key])
+endfunction
+
 function! coc#util#on_error(msg) abort
   echohl Error | echom '[coc.nvim] '.a:msg | echohl None
 endfunction