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. 

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

It compiles

crates/ruff_python_formatter/src/expression/expr_slice.rs

@@ -1,4 +1,5 @@
-use crate::comments::{dangling_comments, Comments, SourceComment};
+use crate::comments::{dangling_comments, SourceComment};
+use crate::context::PyFormatContext;
 use crate::expression::parentheses::{
     default_expression_needs_parentheses, NeedsParentheses, Parentheses, Parenthesize,
 };
@@ -27,8 +28,7 @@ impl FormatNodeRule<ExprSlice> for FormatExprSlice {
             step,
         } = item;
 
-        let (first_colon, second_colon) =
-            find_colons(f.context().contents(), *range, lower, upper)?;
+        let (first_colon, second_colon) = find_colons(f.context().source(), *range, lower, upper)?;
 
         // Handle comment placement
         // In placements.rs, we marked comment for None nodes a dangling and associated all others
@@ -263,9 +263,8 @@ impl NeedsParentheses for ExprSlice {
     fn needs_parentheses(
         &self,
         parenthesize: Parenthesize,
-        source: &str,
-        comments: &Comments,
+        context: &PyFormatContext,
     ) -> Parentheses {
-        default_expression_needs_parentheses(self.into(), parenthesize, source, comments)
+        default_expression_needs_parentheses(self.into(), parenthesize, context)
     }
 }