autocomplete polishing

* suppress autocomplete when navigating through history
* only search for slashcommands if in the first block of the editor
* handle suffix returns from providers correctly
* fix SelectionRange typing in the providers
* fix bugs when pressing ctrl-a, typing and then tab to complete a replacement by collapsing selection to anchor when inserting a completion in the editor
* fix https://github.com/vector-im/riot-web/issues/4762

src/autocomplete/DuckDuckGoProvider.js

@@ -22,6 +22,7 @@ import AutocompleteProvider from './AutocompleteProvider';
 import 'whatwg-fetch';
 
 import {TextualCompletion} from './Components';
+import type {SelectionRange} from './Autocompleter';
 
 const DDG_REGEX = /\/ddg\s+(.+)$/g;
 const REFERRER = 'vector';
@@ -36,7 +37,7 @@ export default class DuckDuckGoProvider extends AutocompleteProvider {
          + `&format=json&no_redirect=1&no_html=1&t=${encodeURIComponent(REFERRER)}`;
     }
 
-    async getCompletions(query: string, selection: {start: number, end: number}) {
+    async getCompletions(query: string, selection: SelectionRange) {
         const {command, range} = this.getCurrentCommand(query, selection);
         if (!query || !command) {
             return [];