gpui: Add SVG rendering to img element and generic asset cache (#9931)

This is a follow up to #9436 . It has a cleaner API and generalized the
image_cache to be a generic asset cache, that all GPUI elements can make
use off. The changes have been discussed with @mikayla-maki on Discord.

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Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collections/src/collections.rs

@@ -10,5 +10,6 @@ pub type HashMap<K, V> = std::collections::HashMap<K, V>;
 #[cfg(not(feature = "test-support"))]
 pub type HashSet<T> = std::collections::HashSet<T>;
 
+pub use rustc_hash::FxHasher;
 pub use rustc_hash::{FxHashMap, FxHashSet};
 pub use std::collections::*;