Variable Font PostScript instance name construction

A variable OpenType font's PostScript name is [supposed to vary](https://adobe-type-tools.github.io/font-tech-notes/pdfs/5902.AdobePSNameGeneration.pdf). This is documented in FreeType by [FT_Get_Postscript_Name](https://gitlab.freedesktop.org/freetype/freetype/-/blob/f42ce25563b73fed0123d18a2556b9ba01d2c76b/include/freetype/freetype.h?page=5#L4555) and implemented in [sfnt_get_var_ps_name](https://gitlab.freedesktop.org/freetype/freetype/-/blob/f42ce25563b73fed0123d18a2556b9ba01d2c76b/src/sfnt/sfdriver.c#L804).

It would be quite useful to provide a means to combine a base PostScript name (from `FontRef::localized_strings(POSTSCRIPT_NAME)`) and a variation position (from `FontRef::axes()`) to create the PostScript instance name. Because of the PostScript name length limit and the non-trivial nature of creating the PostScript instance name it would be best to have this in the library instead of expecting users to re-implement this.

The most specific version of this would be to add a `FontRef::postscript_name() -> Option` like call, or perhaps a more general utility like `postscript_instance_name(LocalizedString, AxisCollection) -> String`. Because there is a length limit on the PostScript name it would be quite awkward to be any more general than this.