This fixes block headers overlapping over text in the buffer when using
a custom line height of 1.25.

It fixes the issue by making the parent container a v-flex,
vertically-justifying the content and moving from relative padding to
absolute padding for the header itself.

## Before/After

With setting:
```json
  "buffer_line_height": {
    "custom": 1.25
  },
```

### Before

![screenshot-2024-01-16-16 48
48@2x](https://github.com/zed-industries/zed/assets/1185253/8c3b977e-333f-403c-a4d3-0911f3fac5e0)

### After

![screenshot-2024-01-16-16 50
13@2x](https://github.com/zed-industries/zed/assets/1185253/2d854eba-c4c4-4bce-b60b-dd250856b08f)

### Release notes
Release Notes:

- Fixed headers in multi-buffers overlapping over content of the buffer