LibGUI: Add Ctrl-U to insert mode

While under insert mode, Ctrl-U deletes all characters between the first
non-blank character of the line and the cursor.

Implement delete_from_line_start_to_cursor() in TextEditor. Then, call
the method in VimEditingEngine via its pointer to an instance of
TextEditor.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -928,6 +928,13 @@ void TextEditor::delete_previous_char()
     execute<RemoveTextCommand>(document().text_in_range(to_erase), to_erase);
 }
 
+void TextEditor::delete_from_line_start_to_cursor()
+{
+    TextPosition start(m_cursor.line(), current_line().first_non_whitespace_column());
+    TextRange to_erase(start, m_cursor);
+    execute<RemoveTextCommand>(document().text_in_range(to_erase), to_erase);
+}
+
 void TextEditor::do_delete()
 {
     if (!is_editable())

Userland/Libraries/LibGUI/TextEditor.h

@@ -137,6 +137,7 @@ public:
     void delete_current_line();
     void delete_previous_word();
     void delete_previous_char();
+    void delete_from_line_start_to_cursor();
     void select_all();
     virtual void undo();
     virtual void redo();

Userland/Libraries/LibGUI/VimEditingEngine.cpp

@@ -802,6 +802,9 @@ bool VimEditingEngine::on_key_in_insert_mode(const KeyEvent& event)
         case KeyCode::Key_H:
             m_editor->delete_previous_char();
             return true;
+        case KeyCode::Key_U:
+            m_editor->delete_from_line_start_to_cursor();
+            return true;
         default:
             break;
         }