ui: Make `Label` respect the `ui_font_weight` setting (#15241)

This PR makes the `Label` component respect the `ui_font_weight`
setting, by default.

An explicit font weight can still be set via the `weight` method, which
will override the `ui_font_weight` for that `Label`.

<img width="1566" alt="Screenshot 2024-07-25 at 8 55 16 PM"
src="https://github.com/user-attachments/assets/2751e29c-c76e-4685-8564-604b3b77f603">

Release Notes:

- Updated UI labels to respect the `ui_font_weight` setting
([#15234](https://github.com/zed-industries/zed/issues/15234)).