Closes https://github.com/zed-industries/zed/issues/41938

For some error messages relating to the keymap file, the font size was
too large. This was due to the error message being a child
`MarkdownString` instead of a `SharedString`. A `.text_xs()` method is
being applied to this notification, but it appears not to affect the
markdown text. I found that the H5 text size in markdown is the same
size as other error messages, so I made each element (that had text)
that size. There was also a special case for bullet points.

I also added a gear icon to the settings button, so it was more in line
with other app notifications.

Error message (text too large):

![keymap-broke](https://github.com/user-attachments/assets/2c205a3a-ae28-419f-95c4-093340760d03)

Expected behavior (notification with correct text sizing and icon):

![keymap-fixed](https://github.com/user-attachments/assets/f8a1396b-177f-4287-b390-c3804b70f1d2)

Example behavior:

![settings](https://github.com/user-attachments/assets/09397954-781f-44be-88ad-08035fe66f0c)

Release Notes:

- Improved UI for keymap error messages.

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>