Syntax highlighting issue when @ appears as first character after line continuation when indexing is present

* **Editor name and version**: VS Code 1.71
* **Platform**: Any
* **Color scheme**: NA
* **MagicPython version**:
* **A sreenshot**:
I would expect syntax highlighting to be the same irrespective of whether the line is broken before or after the @ operator.

* **5-10 lines of surrounding code**:
```python
# Define variables so pylance doesn't complain.
long_variable_name = other_long_variable_name = 1

long_variable_name \
@ other_long_variable_name['some_index']

long_variable_name @ \
other_long_variable_name['some_index']
```

Originally from @tillahoffmann in https://github.com/microsoft/vscode/issues/161435

Reproduce the reported Python snippet in a supported editor, comparing highlighting when the line continuation appears before versus after @ and an indexed expression follows. Trace the syntax-highlighting grammar entry point for line continuations and @ expressions; done means both equivalent forms receive the same highlighting.

Written by the indexing model from the issue text.