Scrollbar markers for cursors (#10816)

How it looks:

https://github.com/zed-industries/zed/assets/2101250/f564111c-1019-4442-b8a6-de338e12b12e

This PR adds cursor markers to the scrollbar. They work similar to
VSCode:

1. A cursor marker takes the whole scrollbar width.
2. It's always 2px high.
3. It uses the player's `cursor` color, so it may be helpful in the
collaboration mode.

There's a setting to switch cursor markers on/off:

```json
{
  "scrollbar": {
    "cursors": true
  }
}
```

Implementation details:

- Unlike other markers, cursor markers are displayed synchronously.
Otherwise they don't feel smooth and sometimes freez on prolonged
up/down navigation.
- Cursor markers are automatically switched off when it's more than 100
of them.
- The minimum (non-cursor) marker height is now 5px. It allows the user
to see other markers under the cursor marker.
- The way the minimum height is imposed on markers has changed a bit to
keep consistency between markers of different types.
- Selected symbol markers use less vibrant color (`info` faded out a
little).

Release Notes:

- Added displaying of cursor markers in the scrollbar. They can be
switched on/off by the `scrollbar.cursors` setting.