Release 0.28.0

Released 2023-07-09.
See also https://rasmus.krats.se/2023/rsass028.en
Progress: 4697 of 7032 tests passed.

• @media rules are now handled specifically as @media rules, rather than as unknown @-rules (PR #172).
• @extend is now explicitly unsupported (gives an error message, rather than behaving as an unknown @ rule) (PR #173).
• Improved handling of @keyframes and @font-face.
• Changed output format of non-finite numbers to match sass spec. They are now wrapped in calc, like calc(infinite) or calc(NaN * 1deg).
• Fixed some difference between module and builtin color functions grayscale and invert.
• The deg unit is no longer spelled out in hsl / hsla colors.
• Improved handling of custom properties in raw css input.
• MSRV of rsass (the lib crate) is now 1.60.0 (required by trancient dev dependency log).
• Improved parsing of quoted strings (some syntax errors was accepted).
• Removed the Error::error(_) shortcut for creating text-only errors (some text only errors remains, created directly as the Error::S(String) variant, that should probably be removed too in the future).
• Fixed a typo in help text, thanks @greyhillman (PR #170).
• Added an input check in selectors.append function.
• Changed Number internals to make values smaller in memory.
• Minor internal cleanup.
• Updated syn in rsass-macros to 2.0.2.
• Updated sass-spec test suite to 2023-06-30.