This PR introduces the `MentionCrease` component, aimed at solving two
issues with mention creases in the agent panel:
- Previously, the mention crease was using a button with a regular size,
which is bigger than the default buffer font line height. That made the
crease look clipped and also overlapping with one another when in a
multiple line scenario where the creases would be on top of each other.
`MentionCrease` uses the window line height value to set the button
height, with a small one pixel vertical padding just for a bit of
spacing.
- Previously, given the crease used a `Label`, its font size wouldn't
scale if you changed the `agent_buffer_font_size` setting. Now,
`MentionCrease` uses that font size value, which makes regular text and
its text grow together as you'd expect.

Release Notes:

- agent: Fix a bug where mention creases didn't scale with
`agent_buffer_font_size` and got clipped/jumbled when rendered one above
the other.