Add methods to iter over f-string elements (#10309)

## Summary

This PR adds methods on `FString` to iterate over the two different kind
of elements it can have - literals and expressions. This is similar to
the methods we have on `ExprFString`.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_ast/src/nodes.rs

@@ -1248,6 +1248,22 @@ pub struct FString {
     pub flags: FStringFlags,
 }
 
+impl FString {
+    /// Returns an iterator over all the [`FStringLiteralElement`] nodes contained in this f-string.
+    pub fn literals(&self) -> impl Iterator<Item = &FStringLiteralElement> {
+        self.elements
+            .iter()
+            .filter_map(|element| element.as_literal())
+    }
+
+    /// Returns an iterator over all the [`FStringExpressionElement`] nodes contained in this f-string.
+    pub fn expressions(&self) -> impl Iterator<Item = &FStringExpressionElement> {
+        self.elements
+            .iter()
+            .filter_map(|element| element.as_expression())
+    }
+}
+
 impl Ranged for FString {
     fn range(&self) -> TextRange {
         self.range