LibLine: Avoid excessive write() syscalls when refreshing the display

Previously, we were generating the display update one character at a
time, and writing them one at a time to stderr, which is not buffered,
doing so caused one syscall per character printed which is s l o w (TM)
This commit makes LibLine write the update contents into a buffer, and
flush it after all the update is generated :^)

Userland/Libraries/LibLine/Editor.h

@@ -383,7 +383,7 @@ private:
     }
 
     void recalculate_origin();
-    void reposition_cursor(bool to_end = false);
+    void reposition_cursor(OutputStream&, bool to_end = false);
 
     struct CodepointRange {
         size_t start { 0 };