CodeEditor does not put squiggles under `squiggle_lines`

It does not do as the document says:
https://github.com/enthought/traitsui/blob/148457a08b4f0fc14ccff6dc2166fa8f0251643c/traitsui/editors/code_editor.py#L105-L107

From what I have tried on qt5, it only adds a small yellow marker next to the scrolling bar. Having squiggles under the lines as it should be can be useful. We want to use it to indicate syntax errors in an application.