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/other/arguments.rs

@@ -36,7 +36,7 @@ impl FormatNodeRule<Arguments> for FormatArguments {
 
         let comments = f.context().comments().clone();
         let dangling = comments.dangling_comments(item);
-        let (slash, star) = find_argument_separators(f.context().contents(), item);
+        let (slash, star) = find_argument_separators(f.context().source(), item);
 
         let format_inner = format_with(|f: &mut PyFormatter| {
             let separator = format_with(|f| write!(f, [text(","), soft_line_break_or_space()]));
@@ -142,7 +142,7 @@ impl FormatNodeRule<Arguments> for FormatArguments {
                 let maybe_comma_token = if ends_with_pos_only_argument_separator {
                     // `def a(b, c, /): ... `
                     let mut tokens =
-                        SimpleTokenizer::starts_at(last_node.end(), f.context().contents())
+                        SimpleTokenizer::starts_at(last_node.end(), f.context().source())
                             .skip_trivia();
 
                     let comma = tokens.next();
@@ -153,7 +153,7 @@ impl FormatNodeRule<Arguments> for FormatArguments {
 
                     tokens.next()
                 } else {
-                    first_non_trivia_token(last_node.end(), f.context().contents())
+                    first_non_trivia_token(last_node.end(), f.context().source())
                 };
 
                 if maybe_comma_token.map_or(false, |token| token.kind() == TokenKind::Comma) {