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/statement/stmt_expr.rs

@@ -13,7 +13,7 @@ impl FormatNodeRule<StmtExpr> for FormatStmtExpr {
 
         if let Some(constant) = value.as_constant_expr() {
             if constant.value.is_str()
-                && !is_expression_parenthesized(value.as_ref().into(), f.context().contents())
+                && !is_expression_parenthesized(value.as_ref().into(), f.context().source())
             {
                 return constant.format().with_options(StringLayout::Flat).fmt(f);
             }