Closes#16993
This PR fixes an issue where the vertical editor scrollbar was
overlaying with buffer headers. I fixed this by reserving space for the
scrollbar as needed which is provided by the recently introduced
`right_margin`.
Most of the diff consists of moving the `EditorMargins` creation out of
`render_block`, as the right margin is stored in this struct and moving
this out reduces the length of the parameter list of `render_blocks` by
one. I thought of this to be a small but nice side effect.
When it comes to the dividers, I decided against these considering the
margin as well, since it felt a bit off. However, I can see arguments
for these also considering the margins. I did include an image for
comparison in the list below. Happy to change this should it be
preferred the other way around.
| `main` |

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| PR |

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| Fix with shortened divider |

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Release Notes:
- Ensured that the vertical editor scrollbar no longer overlaps with
buffer headers.