Make expressions printable (formatting support)

[ajay-mk]

From today's Zoom discussion.

We could introduce overloads for operator<< for Expr or ExprPtr to avoid repeated use of to_latex() everywhere.

[Krzmbrzl]

I propose to instead make the classes work with std::format so we can use C++20's formatting capabilities when printing these objects.

I have already written something like that in https://github.com/ValeevGroup/SeQuant/blob/master/utilities/external-interface/format_support.hpp (though this one works with libfmt instead of the standard library) that could be used as a starting point.