Rust 1.78 (#11314)

Notable things I've had to fix due to 1.78:
- Better detection of unused items
- New clippy lint (`assigning_clones`) that points out places where assignment operations with clone rhs could be replaced with more performant `clone_into`
Release Notes:

- N/A

crates/theme_importer/src/main.rs

@@ -1,6 +1,5 @@
 mod assets;
 mod color;
-mod util;
 mod vscode;
 
 use std::fs::File;