LibGUI: Add Ctrl-H to insert mode

In Vim, Ctrl-H is effectively equivalent to backspace in insert mode, as
it deletes the previous character.

This commit implements method delete_previous_char() to TextEditor.
delete_char() already exists in EditingEngine, but it deletes the
next character rather than the previous. delete_previous_char() is then
called from within VimEditingEngine.

Userland/Libraries/LibGUI/VimEditingEngine.cpp

@@ -799,6 +799,9 @@ bool VimEditingEngine::on_key_in_insert_mode(const KeyEvent& event)
         case KeyCode::Key_W:
             m_editor->delete_previous_word();
             return true;
+        case KeyCode::Key_H:
+            m_editor->delete_previous_char();
+            return true;
         default:
             break;
         }