Refactor to use SharedString in more places (#23813)

Splitting this off from
https://github.com/zed-industries/zed/pull/23808, per @maxdeviant's
suggestion!

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/gpui/src/shared_string.rs

@@ -15,6 +15,11 @@ impl SharedString {
     pub const fn new_static(str: &'static str) -> Self {
         Self(ArcCow::Borrowed(str))
     }
+
+    /// Creates a [`SharedString`] from anything that can become an Arc<str>
+    pub fn new(str: impl Into<Arc<str>>) -> Self {
+        SharedString(ArcCow::Owned(str.into()))
+    }
 }
 
 impl JsonSchema for SharedString {