Make FrameCache::get return a reference instead of cloning the cached value (#7834)

This is a breaking change.

- Enables using `FrameCache` in cases where the cached value cannot be
cloned.
- Improves use cases where only a reference to the cached value is
needed.
- If the user needs an owned value, they can clone it themselves.

Adding a `get_ref` method instead of changing `get` would avoid the
breaking change, but I didn't want to do so because it is kind of
expected for `get` to return `&V` when querying a collection.

crates/egui_extras/src/syntax_highlighting.rs

@@ -116,6 +116,7 @@ fn highlight_inner(
         mem.caches
             .cache::<HighlightCache>()
             .get((&font_id, theme, code, language, settings))
+            .clone()
     })
 }