Allow "console" and "unicode" formats for function units

[olebole]

Description

Originally, function unit format were restricted to generic, unscaled, latex and latex_inline:

>>> import astropy.units as u
>>> print(u.decibel(u.m**-1).to_string('unicode'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/astropy/units/function/core.py", line 395, in to_string
    raise ValueError(
ValueError: Function units cannot be written in unicode format. Only 'generic', 'unscaled', 'latex' and 'latex_inline' are supported.

Also, functional units didn't support the __format__ function:

>>> print(f"{u.decibel(u.m**-1):generic}")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to DecibelUnit.__format__

This PR implements both, including the required implementation of function units with multiline unit format:

>>> import astropy.units as u
>>> print(u.decibel(u.m**-1).to_string('unicode'))
    1
dB( ─)
    m
>>> print(f"{u.decibel(u.m**-1):generic}")
dB(1 / m)
>>> print(f"{u.decibel(u.m**-1):unicode}")
    1
dB( ─)
    m

If #14393 is merged before this, we need to adjust this one a bit depending on the outcome of #14393. Specifically, we could to add **kwargs propagation to the to_string() function to enable/disable multiline.

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[mhvk]

Thanks! This looks good beyond a few nitpicks, but I feel we might as well propagate inline here too -- i.e., get #14393 in first and then rebase. What do you think?

[olebole]

After merging #14393, we get

>>> print(f"{u.decibel(u.m**-1):unicode}")
dB(m⁻¹)
>>> print(f"{u.decibel(u.m**-1):console}")
dB(m^-1)

[mhvk]

Great, thanks!