LibGUI: Soft tab expansion should be based on insertion column

We were using the line length as the column, which worked as long as
you were at the end of the line, but not so well otherwise. :^)

Libraries/LibGUI/TextDocument.cpp

@@ -679,7 +679,7 @@ void InsertTextCommand::perform_formatting(const TextDocument::Client& client)
     const bool should_auto_indent = client.is_automatic_indentation_enabled();
 
     StringBuilder builder;
-    size_t column = dest_line.length();
+    size_t column = m_range.start().column();
     size_t line_indentation = dest_line.leading_spaces();
     bool at_start_of_line = line_indentation == column;