Add new state: Unicode compatibility normalization

Hey ,

I noticed that you are considering only two states:
1. One regarding the path normalization if it is done or not before the safe check
2. Second concerns the safe check.

as shown next:

https://github.com/github/codeql/blob/c1c0a705b9f14c0f577a9ae56a9d699e8b6e67d6/python/ql/lib/semmle/python/security/dataflow/PathInjectionQuery.qll#L20-L28

However, there is a third state that is a required one: `Unicode normalized`. If ever a Unicode normalization is performed with a compatibility algorithm (NFKC or NFKD), the query would miss some cases precisely those ones where the Unicode normalization is not performed before the path normalization and the safe check. I draw a little chart to depict my saying:

The previous chart shows that when you consider a potential Unicode compatibility normalization, it is a required step before path normalization and safe check. If ever placed between the first two steps or after the last one, that would yield a vulnerable case that got missed due to the fact that the Unicode normalization may reintroduce unexpected special characters such as `..` and `/`.

Regards
@Sim4n6

Start in python/ql/lib/semmle/python/security/dataflow/PathInjectionQuery.qll, especially lines 20-28, and compare the existing path-normalization and safe-check states with the reported Unicode compatibility-normalization state. Use the issue’s chart to trace the required ordering; done means the query models compatibility normalization in the relevant position without missing cases.

Scritto dal modello di indicizzazione a partire dal testo della issue.