Pass FormatContext to NeedsParentheses

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## Summary

I started working on this because I assumed that I would need access to options inside of `NeedsParantheses` but it then turned out that I won't. 
Anyway, it kind of felt nice to pass fewer arguments. So I'm gonna put this out here to get your feedback if you prefer this over passing individual fiels. 

Oh, I sneeked in another change. I renamed `context.contents` to `source`. `contents` is too generic and doesn't tell you anything. 

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## Test Plan

It compiles

crates/ruff_python_formatter/src/context.rs

@@ -22,7 +22,7 @@ impl<'a> PyFormatContext<'a> {
         }
     }
 
-    pub(crate) fn contents(&self) -> &'a str {
+    pub(crate) fn source(&self) -> &'a str {
         self.contents
     }