Fix vounit formatting of fractions and powers

[olebole]

Description

Issue #15074 (comment) shows that there are two bugs in the vounit formatte that produce output not compliant to the VOUnits standard. If fractions are used, the formatter adds a space around the slash, and for fractional power exponents the power sign (**) is omitted:

>>> import astropy.units as u
>>> (u.km/u.h).to_string(format="vounit", fraction=True)  # expected: 'km/h'
'km / h'
>>> (u.Hz**0.5).to_string(format="vounit")  # expected: 'Hz**(1/2)'
'Hz(1/2)'
>>> (u.Hz**-0.5).to_string(format="vounit", fraction=True)  # expected: '1/Hz**(1/2)'
'1 / Hz(1/2)'

This PR addresses both, and adds unit tests for them.

[github-actions]

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[mhvk]

Thanks! The code and tests look good! What is still needed is a changelog fragment.

[eerovaher]

This should fit on a single line. Is the trailing comma intentional?

[olebole]

Not sure were it came from, as black had nothing to change here. And IMO trailing commas in this case are normal black format for multi-line parameter lists.

[olebole]

@mhvk Changelog entry is added.

[mhvk]

Looks all good now!