Use g:ignorecase to indicates whether or not ignorecase to matchs completion word (#392)

* Adds `g:lsp_omni_completion_ignorecase` option to indicates whether or not ignorecase to matchs completion word

In most cases, ignorecase is not necessary, so we may add an option to control whether to turn it on.
Defaults, `g:lsp_omni_completion_ignorecase` is `0`, meaning case sensitive, if `g:lsp_omni_completion_ignorecase`
is set to `1`, or set `ignorecase` options(eg. `:set ignorecase`), it will turn case insensitive on.

* Fix CI failed

* Remove `g:lsp_omni_completion_ignorecase` options, only use `g:ignorecase` to indicates whether or not ignorecase to matchs completion word

* Refactor: cache prefix word

Author: zbinlin

autoload/lsp/omni.vim

@@ -63,14 +63,28 @@ function! lsp#omni#complete(findstart, base) abort
             while s:completion['status'] is# s:completion_status_pending && !complete_check()
                 sleep 10m
             endwhile
-            let l:base = tolower(a:base)
-            let s:completion['matches'] = filter(s:completion['matches'], {_, match -> stridx(tolower(match['word']), l:base) == 0})
+            let Is_prefix_match = s:create_prefix_matcher(a:base)
+            let s:completion['matches'] = filter(s:completion['matches'], {_, match -> Is_prefix_match(match['word'])})
             let s:completion['status'] = ''
             return s:completion['matches']
         endif
     endif
 endfunction
 
+function! s:normalize_word(word) abort
+    if &g:ignorecase
+        return tolower(a:word)
+    else
+        return a:word
+    endif
+endfunction
+
+function! s:create_prefix_matcher(prefix) abort
+    let l:prefix = s:normalize_word(a:prefix)
+
+    return { word -> stridx(s:normalize_word(word), l:prefix) == 0 }
+endfunction
+
 function! s:handle_omnicompletion(server_name, complete_counter, data) abort
     if s:completion['counter'] != a:complete_counter
         " ignore old completion results