Fix Indexer fails to identify continuation preceded by newline #10351 (#10354)
## Summary
Fixes #10351
It seems the bug was caused by this section of code
https://github.com/astral-sh/ruff/blob/b669306c87ec304182fd50872b50858e2faf0262/crates/ruff_python_index/src/indexer.rs#L55-L58
It's true that newline tokens cannot be immediately followed by line
continuations, but only outside parentheses. e.g. the exception
```
(
1
\
+ 2)
```
But why was this check put there in the first place? Is it guarding
against something else?
## Test Plan
New test was added to indexer