bug#51324: 28.0.50; Should `message-mark-inserted-region' insert a newline before the end tag?

[Bodertz <bodertz@gmail.com>]

The function `message-mark-inserted-region', bound to {C-c M-m} in
message-mode, inserts "cut here" text around the region.  By default,
the end tag is inserted directly after the text in the region, resulting
in:

    <mark-here-beg-text>
    text in the region<mark-here-end-text>

The text inserted is customizable by changing the values of either or
both of `message-mark-insert-beginning' and `message-mark-insert-end',
so what the defaults are doesn't matter much, but I think it would make
more sense for the default end tag to include a newline at the
beginning, or for the function `message-mark-inserted-region' to insert
a newline before the end tag:

    <mark-here-beg-text>
    text in the region
    <mark-here-end-text>

`message-mark-insert-beginning' already includes a newline at the end,
so I think making `message-mark-insert-end' include a newline at the
beginning makes sense.  Otherwise, a user will have to insert the
newline manually every time, since I don't think the anyone ever wants
to see it without the newline:

    --8<---------------cut here---------------start------------->8---
    text in the region--8<---------------cut here---------------end--------------->8---



The same issue applies when using the prefix argument to use so-called
verbatim marks:

    #v+
    text in the region#v-

This looks wrong to me, but maybe it isn't.