Create a newtype wrapper around Vec<FStringElement> (#11400)

## Summary

This PR adds a newtype wrapper around `Vec<FStringElement>` that derefs
to a `&Vec<FStringElement>`.

Both f-string and format specifier are made up of `Vec<FStringElement>`.
By creating a newtype wrapper around it, we can share the methods for
both parent types.

crates/ruff_python_formatter/src/other/f_string.rs

@@ -128,6 +128,7 @@ impl FStringLayout {
         //
         // Reference: https://prettier.io/docs/en/next/rationale.html#template-literals
         if f_string
+            .elements
             .expressions()
             .any(|expr| memchr::memchr2(b'\n', b'\r', locator.slice(expr).as_bytes()).is_some())
         {