fix: Avoid restarting autocompletion after selecting option

Debouncing ensures the native suggestion UI does not reappear after
selecting an suggestion.

Author: dcalhoun

src/components/editor/use-at-autocompleter.js

@@ -52,8 +52,7 @@ function addAtSymbolCompleter( completers = [] ) {
 			// Return empty array since we're not providing actual completion options
 			return [];
 		},
-		getOptionLabel: () => '',
-		getOptionCompletion: () => '@',
+		isDebounced: true,
 	};
 
 	return [ ...completers, atSymbolCompleter ];

src/components/editor/use-plus-autocompleter.js

@@ -46,8 +46,7 @@ function addPlusSymbolCompleter( completers = [] ) {
 			// Return empty array since we're not providing actual completion options
 			return [];
 		},
-		getOptionLabel: () => '',
-		getOptionCompletion: () => '+',
+		isDebounced: true,
 	};
 
 	return [ ...completers, plusSymbolCompleter ];