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/client/src/client.rs

@@ -1752,7 +1752,7 @@ impl CredentialsProvider for KeychainCredentialsProvider {
 }
 
 /// prefix for the zed:// url scheme
-pub static ZED_URL_SCHEME: &str = "zed";
+pub const ZED_URL_SCHEME: &str = "zed";
 
 /// Parses the given link into a Zed link.
 ///