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/project/src/lsp_command/signature_help.rs

@@ -86,7 +86,7 @@ impl SignatureHelp {
         } else {
             let markdown = markdown.join(str_for_join);
             let language_name = language
-                .map(|n| n.name().0.to_lowercase())
+                .map(|n| n.name().as_ref().to_lowercase())
                 .unwrap_or_default();
 
             let markdown = if function_options_count >= 2 {