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

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