Add support to skip setting buffer omnifunc

Close #1079

Author: autozimu

doc/LanguageClient.txt

@@ -589,6 +589,13 @@ Hides all virtual texts for the current buffer while editing in insert mode.
 Default: 0
 Valid options: 1 | 0
 
+2.38 g:LanguageClient_setOmnifunc                *g:LanguageClient_setOmnifunc*
+
+Whether set buffer omnifunc to 'LanguageClient#complete'.
+
+Default: v:true
+Valid options: v:true | v:false
+
 ==============================================================================
 3. Commands                                           *LanguageClientCommands*
 

src/language_server_protocol.rs

@@ -2184,6 +2184,9 @@ impl LanguageClient {
         let filename = self.vim()?.get_filename(params)?;
         let language_id = self.vim()?.get_language_id(&filename, params)?;
         let text = self.vim()?.get_text(&filename)?;
+        let set_omnifunc: bool = self
+            .vim()?
+            .eval("s:GetVar('LanguageClient_setOmnifunc', v:true)")?;
 
         let text_document = TextDocumentItem {
             uri: filename.to_url()?,
@@ -2203,8 +2206,10 @@ impl LanguageClient {
             DidOpenTextDocumentParams { text_document },
         )?;
 
-        self.vim()?
-            .command("setlocal omnifunc=LanguageClient#complete")?;
+        if set_omnifunc {
+            self.vim()?
+                .command("setlocal omnifunc=LanguageClient#complete")?;
+        }
         let root = self.get(|state| state.roots.get(&language_id).cloned().unwrap_or_default())?;
         self.vim()?.rpcclient.notify(
             "setbufvar",