Implement the man(7) .MR macro, a 2023 GNU extension.

Implement the man(7) .MR macro, a 2023 GNU extension.
The syntax and semantics is almost identical to mdoc(7) .Xr.

This will be needed for reading the groff manual pages once our port
will be updated to 1.23, and the Linux Manual Pages Project is also
determined to start using it sooner or later.  I did not advocate for
this new macro, but since we want to remain able to read all manual
pages found in the wild, there is little choice but to support it.
At least it is easy to do, they basically copied .Xr.