Add analyzer that flags var in pattern matching

Based on internal discussion.
Pattern matching with var provides little value since it effectively matches everything and ends up copying to a local value.

Match on this
```
if ( bar is var foo )
```

Suggested code change (just to help educate people)
```
var foo = bar;
if ( true )
```

No files, tests, or analyzer entry points are named in the issue. First locate the repository's analyzer implementation and test layout, then use the var-in-pattern example as the reproduction; done means the analyzer flags this pattern and the behavior is covered by tests.

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