Add a derive macro for Combine (#4325)

## Summary

Saves us some boilerplate when adding settings in the future.

crates/uv-macros/src/lib.rs

@@ -0,0 +1,40 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(CombineOptions)]
+pub fn derive_combine(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+    impl_combine(&input)
+}
+
+fn impl_combine(ast: &DeriveInput) -> TokenStream {
+    let name = &ast.ident;
+    let fields = if let syn::Data::Struct(syn::DataStruct {
+        fields: syn::Fields::Named(ref fields),
+        ..
+    }) = ast.data
+    {
+        &fields.named
+    } else {
+        unimplemented!();
+    };
+
+    let combines = fields.iter().map(|f| {
+        let name = &f.ident;
+        quote! {
+            #name: self.#name.combine(other.#name)
+        }
+    });
+
+    let gen = quote! {
+        impl crate::Combine for #name {
+            fn combine(self, other: #name) -> #name {
+                #name {
+                    #(#combines),*
+                }
+            }
+        }
+    };
+    gen.into()
+}