Quantity.to_string() should allow unicode format #14386

What is the problem this feature will solve?

Currently, units can be typeset in unicode, but Quantity cannot. However, Angle.to_string does support unicode.

Describe the desired outcome

That the .to_string() methods of Unit, Quantity, and all Quantity subclasses such as Angle support as much as possible the same arguments.

Additional context

See #14379

EDIT: A tricky bit is that for inline=False, i.e., when the units are typeset as a fraction, one needs to add the correct number of spaces, otherwise one still gets what used to happen with __format__ (#14398):

import astropy.units as u
q = 10*u.m/u.s
print(f"{q:unicode}")
10.0  m
 ─
 s

Also not obvious is what should happen for the case that the quantity is an array; but at least inline=True should be possible.

EDIT (2024-01-11): the :unicode formatting now typesets the unit in-line, so it is less broken, though numbers in arrays are not typeset using unicode, unlike for latex.

10.0 m s⁻¹