gpui: Add rounded_md token (#26179)

This PR adds a new rounded/corner border token: `rounded_md` with a
value of 6px.

I feel like I was wanting to use 6px border radius a lot but avoiding
due to it being an arbitrary value... so, not anymore! It's also cool to
have this be consistent with Tailwind v4.

Follow on to the prior renames:

- `rounded_sm` -> `rounded_xs`:
https://github.com/zed-industries/zed/pull/26221
- `rounded_md` -> `rounded_sm`:
https://github.com/zed-industries/zed/pull/26228

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/gpui_macros/src/styles.rs

@@ -1242,6 +1242,11 @@ fn corner_suffixes() -> Vec<CornerStyleSuffix> {
             radius_tokens: quote! { rems(0.25) },
             doc_string_suffix: "4px (0.25rem)",
         },
+        CornerStyleSuffix {
+            suffix: "md",
+            radius_tokens: quote! { rems(0.375) },
+            doc_string_suffix: "6px (0.375rem)",
+        },
         CornerStyleSuffix {
             suffix: "lg",
             radius_tokens: quote! { rems(0.5) },