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/editor/src/editor.rs

@@ -6550,7 +6550,7 @@ impl Editor {
             .pl_1()
             .pr(padding_right)
             .gap_1()
-            .rounded(px(6.))
+            .rounded_md()
             .border_1()
             .bg(Self::edit_prediction_line_popover_bg_color(cx))
             .border_color(Self::edit_prediction_callout_popover_border_color(cx))