Use const over static for string literals (#18635)

I noticed a few places where we were storing `&'static str`s in
`static`s instead of `const`s.

This PR updates them to use `const`.

Release Notes:

- N/A

crates/gpui/src/taffy.rs

@@ -24,7 +24,7 @@ pub struct TaffyLayoutEngine {
     nodes_to_measure: FxHashMap<LayoutId, NodeMeasureFn>,
 }
 
-static EXPECT_MESSAGE: &str = "we should avoid taffy layout errors by construction if possible";
+const EXPECT_MESSAGE: &str = "we should avoid taffy layout errors by construction if possible";
 
 impl TaffyLayoutEngine {
     pub fn new() -> Self {