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

还没有人认领这个 Issue。

* **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

在受支持的编辑器中重现报告的 Python 代码片段，比较行延续出现在 @ 之前和之后、且后面跟随索引表达式时的高亮效果。追踪行延续和 @ 表达式的语法高亮语法入口点；完成标准是两种等价形式都获得相同的高亮效果。

由索引模型根据 Issue 内容生成。